PHP
Rather surprising to see PHP so high here since I rarely see PHP-related submissions on HN. How do people learn it? I've found that the best thing for me was finding StumbleUpon and reading up as much as I could about best practices in PHP and programming in general. Now I force myself to try a different language for my own projects to get away from PHP as a "comfort zone".
I've since read two of Ullman's books (PHP for the WWW and PHP 5 Advanced), the Powers books (PHP Solutions and PHP Object-Oriented Solutions), and Shiflett's 'Essential PHP Security' (the PHPSec guide is required reading also).
The two Apress Pro books ('PHP Objects, Patterns and Practice' and 'Pro PHP'), 'Essential PHP Tools', 'PHP5 Power Programming' (Andi Gutmans is a coauthor) and 'Advanced PHP Programming' by Schlossnagle are up next, although I'm currently working through SICP.
(Note: I switched to Postgres since reading Welling & Thomson; there are basically 10x as many MySQL books as PostgreSQL ones, unfortunately)