1) Maintain compatibility with URLS that don't specify a virtual library.

2) Documentation.

manual/url_scheme.rst

@@ -56,6 +56,18 @@ brackets at the end of the path to the book folder.
 You can copy a link to the current book displayed in calibre by right clicking
 the :guilabel:`Book details` panel and choosing :guilabel:`Copy link to book`.
 
+If a Virtual library is selected, calibre will use it when showing the book. If
+the book isn't found in that virtual library then the virtual library is cleared.
+
+If you want to switch to a particular Virtual library when showing the book, use::
+
+    calibre://show-book/Library_Name/book_id?virtual_library=Library%20Name
+    or
+    calibre://show-book/Library_Name/book_id?encoded_virtual_library=hex_encoded_virtual_library_name
+
+replacing spaces in the Virtual library name by ``%20``. If the book isn't in that
+virtual library then it is ignored.
+
 
 Open a specific book in the E-book viewer at a specific position
 -------------------------------------------------------------------

src/calibre/gui2/ui.py

@@ -707,7 +707,8 @@ class Main(MainWindow, MainWindowMixin, DeviceMixin, EmailMixin,  # {{{
             vl = get_virtual_library(query)
 
             def doit():
-                if vl != '_':
+                # To maintain compatibility, don't change the VL if it isn't specified.
+                if vl is not None and vl != '_':
                     self.apply_virtual_library(vl)
                 rows = self.library_view.select_rows((book_id,))
                 db = self.current_db