I have several Visio books - most are useless for developers (people who write VBA/VB/VC++ code for Visio automation). For those who develop for Visio, this is a must-have book. If you're using 2002 or 2003, the same author has newer versions of the book for those. He also lets you download software tools to use - one awesome one is a VB DLL/VSL adapter. But even without the extra software this book goes into more detail and depth than any other, except the Microsoft Devloper book (which is free online). And it's a great companion to the Msoft book, and covers things it doesn't. (including highlighting the bugs)
All in all, an excellent book.