I would check out the line of books by O'Reilly Media, and subscribe to their RSS feeds for daily specials. I have bought a few of their e-books when they are on sale.
I have also had success combing the "remainder" shelves at the bookstores for deep discounts on the "last version" books. Getting a $65 list book on PHP for $5, even if it is a version behind, makes me happy. You have to supplement any printed resource with an on-line query to make sure some weird combination of your installation don't cause conflicts (did you know that the PHP cache APC doesn't play well with SuEXEC? I do now.)
BTW - for MySQL tuning, try getting "mysqltuner.pl" (you can wget it from root) and running it. It will give you recommendations for editing your my.cnf file to optimize MySQL.