This book was laid out very nicely and covered all the topics a developer needs to know in order to create Internet-enabled applications. And I agree with the other reviewers that it may be the best book on WinInet (isn't it the ONLY book?), can be used as a reference guide, and the asynchronous section was insightful.
However, for a developer who is trying to take these APIs in a new direction (really do some hard-core ATL with C++) with COM, I found the book a bit lacking. [Admittedly, this book was focused on applications, not objects]. I needed more information about creating UI-less COM objects and perhaps I was looking for this book to do more than it claims. For example, the pre-IE5 WinInet API set doesn't support UNICODE and that information is key to C++ COM developers.
Definitive Guide? Not quite.
If you're looking to do some hard-core COM development with WinInet, I'd stick with the WinInet Reference on Microsoft's site, grab a good ATL reference and go from there (espeically if you're someone who can pick up a new, relatively small API set quickly. There's really nothing to WinInet).
If you're just using VB to create your COM objects or application, then this book is a good resource.