diff --git a/src/calibre/manual/develop.rst b/src/calibre/manual/develop.rst
index c49176ceb2..7068bc2389 100644
--- a/src/calibre/manual/develop.rst
+++ b/src/calibre/manual/develop.rst
@@ -36,10 +36,13 @@ Code layout
 All the |app| python code is in the ``calibre`` package. This package contains the following main sub-packages
 
     * devices - All the device drivers. Just look through some of the built-in drivers to get an idea for how they work.
+      * For details, see: devices.interface which defines the interface supported by device drivers and devices.usbms which defines a generic driver that connects to a USBMS device. All USBMS based drivers in calibre inherit from it.
     * ebooks  - All the ebook conversion code. A good starting point is ``calibre.ebooks.conversion.cli`` which is the
-      module powering the :command:`ebook-convert` command.
-    * library - The database backed and the content server. 
-    * gui2 - The Graphical User Interface. 
+      module powering the :command:`ebook-convert` command. The conversion process is controlled via conversion.plumber.
+      The format independent code is all in ebooks.oeb and the format dependent stuff is in ebooks.format_name.
+        * Metadata reading writing and downloading is all in ebooks.metadata
+    * library - The database backed and the content server. See library.database2 for the interface to the calibre library. library.server is the calibre Content Server.
+    * gui2 - The Graphical User Interface. GUI initialization happens in gui2.main and gui2.ui. The ebook-viewer is in gui2.viewer.
 
 Getting the code
 ------------------