OpenCAD - how to write a CAD program

The "OpenCAD - A Step by Step Guide to Developing a Professional CAD Application" book by Deelip Menezes describes in a single book how (with the assistance of the DWGdirect SDK. Focused on a single library and on Windows applications using MFC, but interesting nonetheless.

PDF and physical copies are available.

http://www.sycode.com/publications/books/opencad/