…is that they are too important to take time for the grunge work that makes a technical book useful.
Agile Web Development with Rails is essentially a book-length tutorial, and imho not worth either the money or time. The entire book is narrative. It admits as much in the introduction: “This book isn’t meant to be a reference manual for Rails…reference manuals are not the way most people learn.” Maybe so, but they are extremely helpful productivity tools when trying to do actual work! (The narrative itself is so patronizing and self-aggrandizing its painful to plow thru, though probably does have some value if you can stomach it.) An auto-generated code API is no replacement for a human edited reference manual with context. It seems Rails requires you to ‘read the code not the docs’ in order to do anything nonstandard, which works, but…not exactly productivity enhancing as advertised.
Browsing a good reference lets you see immediately the structure of a framework.
PHP.net is beautiful and so efficient, I’ve never bought a PHP book either – but for the opposite reason I’d advise not buying this one.