Don't let the modern-looking cover fool you. This book has been around for almost every version of Visio, and has been continually updated for every subsequent product release.
This is the quintessential reference book for Visio developers. A complete ShapeSheet and Object Model reference, this book will cover every object, method, property, event, and ShapeSheet formula you would ever want to know. In particular, much attention is paid to creating "SmartShapes", meaning using less "code", but taking advantage of the ShapeSheet and it's power to create highly intelligent Visio shapes. While most coders jump right in to coding, they do not realize that in Visio, good applications start with "shapes first", then "code later".
This book doesn't cover automating Visio's extra dll's and vsl's, like the UML, Organizational Chart, and Database Modeling solutions, but those add-ons are not really part of Visio's native object model (you'll have to dig a little harder for info of those).