Great book! I used it extensively while creating the Add-In for CodeHound.com. This book is really the only reference you'll find outside the (meager) VB help files for creating a VB Add-In.

Mistakes in an Add-In come at a heavy cost - the stability of the users' development environment. If you're going to create an Add-In for release to the general public, you definitely want to read all the references you can get your hands on.