Pro Jakarta Struts, Second Edition by John Carnell with Rob Harrop is the best book on developing Struts-based web applications I've read. (I've read at least five or six others.)
The title of the book is a bit of a misnomer, as it covers much more than just Struts 1.1 fundamentals. This book aims higher, including chapters on web application Patterns and Antipatterns, XDoclet, Velocity and ObjectRelationalBridge, along with the good explanations of various Struts-specific topics.
Pro Jakarta Struts, Second Edition is a pretty thick book at 578 pages. I didn't notice a single page wasted to printing some API. It isn't really a technical cookbook to be placed on a nearby shelf for quick reference during development to look up the basics of using some technology. Instead, the authors focused on good practices used when developing web applications, including explanations of why a technology should be used and how to use it, recurring problems that others have experienced (Antipatterns) and what to do instead (Patterns), and other pieces of advice designed to get the reader's brain thinking in web applications.
If you're in the market for a book on Struts-based web applications, get this one. It covers Struts 1.1 well, but it doesn't stop where Struts leaves off.