I think 50% of the quality of a tech book is in the index and the table of contents (in that order). If I can't find it in the index, it may as well not exist in the book -- I'm certainly not going to spend an hour proving the index is correct.
I've ranted elsewhere about specific things not being in the Camel index, which turns out were mentioned in the text. I'm not picking on that one, many are like that. In fact, like anything else, there are very few "great" indices.
After I had the basic syntax worked out, things like Effective Perl Programming took me further than any reference book. Though I've settled on the Camel book to look up functions. But I would have really liked a printed, indexed version of perldoc in the early days. Now, online access is much more prevalent, so it's not a hardship to go looking. But when I'm stuck in a DOS screen trying to read perldoc perlop...well, don't go there, I'm still annoyed with it.
Quantum Mechanics: The dreams stuff is made of