A book which I don't often come back to but I'm just about to read through again (it's not really very usefull as a reference book) and I think is an incredibly helpfull text is Oreilly's Mastering Regular Expressions.
Second on my list is a book which has been an excellent reference for everything from DTDs to Schemas, CSS to XSL-FO and all the rest in the world of XML. I've only had the opporunity to make use of it's knowledge in my tiny little apps so far but I can't see myself putting this bok down for more than two weeks for a long time. It's Oreilly's XML In A Nutshell
Last but by no means least is a book which I have found invaluable for about three years now. It's a Linux guide by the renowned (apparently, I'd never heard of him before) Unix author Mark Sobell. It's divided into two sections, a tutorial based guide to start with and a reference to all the most common Linux commands for seconds. It's A Practical Guide to Linux
I have recently ordered Oreilly's PHP Cookbok on Piersk's recomendation (so I know who I'll be comming after if it's not up to scratch ) and pre-ordered Oreilly's Upgrading to PHP5. I'll let Piersk sin the praises of the former as he did it so well and let you know how the latter turns out when I'm done with it.