diff --git a/src/pyj/read_book/iframe.pyj b/src/pyj/read_book/iframe.pyj
index 649a5a4ebb..cd47ecabe1 100644
--- a/src/pyj/read_book/iframe.pyj
+++ b/src/pyj/read_book/iframe.pyj
@@ -29,6 +29,7 @@ from read_book.paged_mode import (
     scroll_by_page as paged_scroll_by_page, scroll_to_elem,
     scroll_to_fraction as paged_scroll_to_fraction, snap_to_selection
 )
+from read_book.referencing import end_reference_mode, start_reference_mode
 from read_book.resources import finalize_resources, unserialize_html
 from read_book.settings import (
     apply_colors, apply_font_size, apply_settings, apply_stylesheet, opts,
@@ -80,6 +81,7 @@ class IframeBoss:
     def __init__(self):
         window.navigator.epubReadingSystem = EPUBReadingSystem()
         self.last_cfi = None
+        self.reference_mode_enabled = False
         self.replace_history_on_next_cfi_update = True
         self.blob_url_map = {}
         self.content_ready = False
@@ -100,6 +102,7 @@ class IframeBoss:
             'window_size': self.received_window_size,
             'get_current_cfi': self.get_current_cfi,
             'set_forward_keypresses': self.set_forward_keypresses,
+            'set_reference_mode': self.set_reference_mode,
         }
         self.comm = IframeClient(handlers)
         self.last_window_ypos = 0
@@ -155,6 +158,7 @@ class IframeBoss:
         self.content_ready = False
         self.replace_history_on_next_cfi_update = True
         self.book = current_book.book = data.book
+        self.reference_mode_enabled = data.reference_mode_enabled
         self.is_titlepage = data.is_titlepage
         spine = self.book.manifest.spine
         index = spine.indexOf(data.name)
@@ -178,7 +182,13 @@ class IframeBoss:
             self.anchor_funcs = paged_anchor_funcs
         update_settings(data.settings)
         self.keyboard_shortcut_map = create_shortcut_map(data.settings.keyboard_shortcuts)
-        set_current_spine_item({'name':data.name, 'is_first':index is 0, 'is_last':index is spine.length - 1, 'initial_position':data.initial_position})
+        set_current_spine_item({
+            'name':data.name,
+            'is_first':index is 0,
+            'is_last':index is spine.length - 1,
+            'index': index,
+            'initial_position':data.initial_position
+        })
         self.last_cfi = None
         for name in self.blob_url_map:
             window.URL.revokeObjectURL(self.blob_url_map[name])
@@ -239,6 +249,8 @@ class IframeBoss:
         if self.is_titlepage and not opts.cover_preserve_aspect_ratio:
             document.body.classList.add('cover-fill')
         document.body.classList.add(f'calibre-viewer-{layout_style()}')
+        if self.reference_mode_enabled:
+            start_reference_mode()
         self.last_window_width, self.last_window_height = scroll_viewport.width(), scroll_viewport.height()
         apply_settings()
         fix_fullscreen_svg_images()
@@ -269,8 +281,7 @@ class IframeBoss:
                 self.find(ipos.search_data, True)
         spine = self.book.manifest.spine
         files = self.book.manifest.files
-        current_name = current_spine_item().name
-        spine_index = spine.indexOf(current_name)
+        spine_index = csi.index
         self.length_before = 0
         if spine_index > -1:
             for i in range(spine_index):
@@ -297,9 +308,7 @@ class IframeBoss:
         cfi = current_cfi()
         selected_text = window.getSelection().toString()
         if cfi:
-            spine = self.book.manifest.spine
-            current_name = current_spine_item().name
-            index = spine.indexOf(current_name)
+            index = current_spine_item().index
             if index > -1:
                 cfi = 'epubcfi(/{}{})'.format(2*(index+1), cfi)
                 self.send_message(
@@ -312,9 +321,7 @@ class IframeBoss:
     def update_cfi(self):
         cfi = current_cfi()
         if cfi:
-            spine = self.book.manifest.spine
-            current_name = current_spine_item().name
-            index = spine.indexOf(current_name)
+            index = current_spine_item().index
             if index > -1:
                 cfi = 'epubcfi(/{}{})'.format(2*(index+1), cfi)
                 pf = self.calculate_progress_frac()
@@ -472,6 +479,16 @@ class IframeBoss:
             else:
                 self.send_message('find_in_spine', text=data.text, backwards=data.backwards, searched_in_spine=data.searched_in_spine)
 
+    def reference_item_changed(self, ref_num_or_none):
+        self.send_message('reference_item_changed', refnum=ref_num_or_none, index=current_spine_item().index)
+
+    def set_reference_mode(self, data):
+        self.reference_mode_enabled = data.enabled
+        if data.enabled:
+            start_reference_mode()
+        else:
+            end_reference_mode()
+
 
 def main():
     main.boss = IframeBoss()
diff --git a/src/pyj/read_book/referencing.pyj b/src/pyj/read_book/referencing.pyj
new file mode 100644
index 0000000000..ad437ada20
--- /dev/null
+++ b/src/pyj/read_book/referencing.pyj
@@ -0,0 +1,35 @@
+# vim:fileencoding=utf-8
+# License: GPL v3 Copyright: 2019, Kovid Goyal <kovid at kovidgoyal.net>
+# noqa: eol-semicolon
+from __python__ import bound_methods, hash_literals
+
+from read_book.globals import get_boss
+
+
+def on_mouse_over(ev):
+    p = this
+    if p.dataset.calibreRefNum:
+        refnum = int(p.dataset.calibreRefNum)
+        get_boss().reference_item_changed(refnum)
+
+
+def on_mouse_out(ev):
+    get_boss().reference_item_changed(None)
+
+
+def start_reference_mode():
+    i = 0
+    for p in document.getElementsByTagName('p'):
+        i += 1
+        if not p.dataset.calibreRefNum:
+            p.dataset.calibreRefNum = i + ''
+            p.addEventListener('mouseover', on_mouse_over, {'passive': True})
+            p.addEventListener('mouseout', on_mouse_out, {'passive': True})
+
+
+def end_reference_mode():
+    for p in document.getElementsByTagName('p'):
+        if p.dataset.calibreRefNum:
+            p.removeEventListener('mouseover', on_mouse_over, {'passive': True})
+            p.removeEventListener('mouseout', on_mouse_out, {'passive': True})
+            v'delete p.dataset.calibreRefNum'
diff --git a/src/pyj/read_book/shortcuts.pyj b/src/pyj/read_book/shortcuts.pyj
index 11569e8100..efd4c3e112 100644
--- a/src/pyj/read_book/shortcuts.pyj
+++ b/src/pyj/read_book/shortcuts.pyj
@@ -228,6 +228,12 @@ def shortcuts_definition():
         _('Toggle between Paged mode and Flow mode for text layout')
     ),
 
+    'toggle_reference_mode': desc(
+        'Ctrl+x',
+        'ui',
+        _('Toggle the Reference mode')
+    ),
+
     'reload_book': desc(
         v"['F5', 'Ctrl+r']",
         'ui',
diff --git a/src/pyj/read_book/view.pyj b/src/pyj/read_book/view.pyj
index bba43543c7..68d6b79efa 100644
--- a/src/pyj/read_book/view.pyj
+++ b/src/pyj/read_book/view.pyj
@@ -147,6 +147,7 @@ class View:
 
     def __init__(self, container):
         self.timers = Timers()
+        self.reference_mode_enabled = False
         self.loaded_resources = {}
         self.current_progress_frac = self.current_file_progress_frac = 0
         self.current_toc_node = self.current_toc_toplevel_node = None
@@ -190,8 +191,13 @@ class View:
                         E.div(style='position: absolute; top:0; left:0; width: 100%; height: 100%; overflow: auto; display:none', id='book-overlay'),  # main overlay
                         E.div(style='position: absolute; top:0; left:0; width: 100%; height: 100%; display:none', id='controls-help-overlay'),  # controls help overlay
                     )
-                )
-            )
+                ),
+                E.div(
+                    style=f'color: {get_color("window-foreground")}; background: {get_color("window-background")};' +
+                    'position: absolute; display: none; left: 0; top: 0; padding: 0.5ex; border: solid 2px; z-index: 3000',
+                    id='reference-mode-overlay'
+                ),
+            ),
         )
         handlers = {
             'ready': self.on_iframe_ready,
@@ -216,6 +222,7 @@ class View:
             'selectionchange': self.on_selection_change,
             'handle_shortcut': self.on_handle_shortcut,
             'handle_keypress': self.on_handle_keypress,
+            'reference_item_changed': self.on_reference_item_changed,
         }
         entry_point = None if runtime.is_standalone_viewer else 'read_book.iframe'
         if runtime.is_standalone_viewer:
@@ -236,6 +243,10 @@ class View:
     def iframe(self):
         return self.iframe_wrapper.iframe
 
+    @property
+    def reference_mode_overlay(self):
+        return document.getElementById('reference-mode-overlay')
+
     def on_lookup_word(self, data):
         if runtime.is_standalone_viewer:
             ui_operations.selection_changed(data.word)
@@ -366,6 +377,8 @@ class View:
             self.bump_font_size({'increase': False})
         elif data.name is 'toggle_full_screen':
             ui_operations.toggle_full_screen()
+        elif data.name is 'toggle_reference_mode':
+            self.toggle_reference_mode()
         elif data.name is 'reload_book':
             ui_operations.reload_book()
         elif data.name is 'search_for_selection':
@@ -456,6 +469,7 @@ class View:
         self.overlay.hide()
         self.search_overlay.hide()
         self.content_popup_overlay.hide()
+        self.reference_mode_overlay.style.display = 'none'
         self.focus_iframe()
 
     def focus_iframe(self):
@@ -932,7 +946,7 @@ class View:
         self.iframe_wrapper.send_unencrypted_message('display',
             resource_data=resource_data, book=self.book, name=self.currently_showing.name,
             initial_position=self.currently_showing.initial_position,
-            settings=self.currently_showing.settings,
+            settings=self.currently_showing.settings, reference_mode_enabled=self.reference_mode_enabled,
             is_titlepage=self.currently_showing.name is self.book.manifest.title_page_name,
         )
 
@@ -941,6 +955,7 @@ class View:
         self.hide_loading()
         self.set_progress_frac(data.progress_frac, data.file_progress_frac)
         self.update_header_footer()
+        self.on_reference_item_changed()
         window.scrollTo(0, 0)  # ensure window is at 0 on mobile where the navbar causes issues
         if self.book_load_started:
             self.book_load_started = False
@@ -958,3 +973,17 @@ class View:
     def update_color_scheme(self):
         cs = self.get_color_scheme(True)
         self.iframe_wrapper.send_message('change_color_scheme', color_scheme=cs)
+
+    def toggle_reference_mode(self):
+        self.reference_mode_enabled = not self.reference_mode_enabled
+        self.iframe_wrapper.send_message('set_reference_mode', enabled=self.reference_mode_enabled)
+
+    def on_reference_item_changed(self, data):
+        data = data or {'refnum': None, 'index': None}
+        refnum, index = data.refnum, data.index
+        div = self.reference_mode_overlay
+        if refnum is None:
+            div.style.display = 'none'
+        else:
+            div.style.display = 'block'
+            div.textContent = f'{index}.{refnum}'