Learning new programming languages is fun but it can be difficult choosing which references and books to use. Most good text book will cost a small fortune and go out of date within a few months and probably wont cover everything you need.
For me the best resource comes straight from the source, for most languages you will be able to find a FREE and complete reference to develop with.
PHP Documentation – Download the complete reference in most spoken languages.
w3 CSS v1 – Full guide to CSS v1 from 1996 – 2009 in PDF format.
w3 guide to css 2.1 – Full on-line reference to css v2.1
w3 HTML 3 guide - An awesome guide to HTML, very handy for SEO Tricks,
Action Script 3 – Official full guide to Action script 3 programming.
Mysql - One of the less helpful guides but it is fairly complete.
WordPress Functions - The official function list, awesome for any wordpress project