Web development
Designing for Web Standards
This book has been sorely needed. Zeldman, publisher of A List Apart tackles the topic of forward compatability.
CSS: Separating Content from Presentation
An excellent book on the strategy of applying CSS to the design of Web sites. There are plenty of reference books, but precious few on the rationale for solving design problems.
Cascading Style Sheets: The Definitive Guide
Eric Meyer is one of the foremost experts on CSS. This book covers the spec, as opposed to strategies, tutorials, or canned layouts. An excellent resource.
Dynamic HTML: The Definitive Reference
If you judge books based on the number of pages, you're in for a treat. This book is a great, and very large, resource.
PHP and MySQL Web Development
This book is extremely well written and approachable. It really stands out from the rest of books that are seemingly written by people who have never set foot outside their computer lab.
Using HTML and XHTML
A good reference on XHTML. Most of it is available online, but a book is much quicker in a debugging crisis.