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Essential Web Development Knowledge (O'Reilly Style)
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Dojo: The Definitive GuideDojo: The Definitive Guide by Matthew A. Russell
Buy new: $26.39 / Used from: $12.62
The book I wrote on Dojo because there was simply too much breadth and depth in this fantastic toolkit to possibly keep it all straight without a little bit of help. I'll be as excited as anyone else to get a printed paper copy on my desk for so I can mark it all up, stick in bookmarks, etc. (I'm old fashioned like that.)
JavaScript Definitive GuideJavaScript Definitive Guide by David Flanagan
Buy used from: $3.68
The authoritative reference on JavaScript. Whenever I need to look up some of the truly esoteric parts of the language, this is the place I always turn to. The API reference in the second half is terrific, and I can usually find the info I'm looking for quicker in this printed book than searching online.
Cascading Style Sheets: The Definitive Guide, 2nd EditionCascading Style Sheets: The Definitive Guide, 2nd Edition by Eric A. Meyer
Buy used from: $2.73
Let's face it: very few of us have truly mastered CSS or the nuances of how different browsers implement the box model, standards mode vs quirks mode, etc. This book will save you a lot of time when it comes time to get your CSS game on.
Restful Web ServicesRestful Web Services by Leonard Richardson
Buy new: $26.39 / Used from: $2.48
A fantastic overview of just about anything you'd ever want to know about designing a RESTful architecture. On the surface, GETting vs POSTing, etc. might all seem pretty obvious, but in practice there are always some grey areas that come up. This book did a great job of helping me to get many of those finer points straight.
Ajax: The Definitive GuideAjax: The Definitive Guide by Anthony T. Holdener III
Buy new: $31.49 / Used from: $6.07
A tremendous work on AJAX from a toolkit-agnostic standpoint that covers a ton of use cases and common designs that you will definitely find yourself coming across. While, I generally prefer doing actual development with Dojo, it's also nice to have a more general purpose reference on some of the fundamentals within arm's reach.
Essential ActionScript 3.0 (Essential)Essential ActionScript 3.0 (Essential) by Colin Moock
Buy new: $34.64 / Used from: $20.98
This is one of the most exhaustive references on any topic that I've ever seen. If you even need to so much as dabble in Flex, Flash, AIR, or any other Adobe-based web technology, having this book nearby is a must--especially if you don't consider yourself an ActionScript expert and sometimes need to quickly find definitive knowledge on topics that you really shouldn't be guessing about.
Programming Collective Intelligence: Building Smart Web 2.0 ApplicationsProgramming Collective Intelligence: Building Smart Web 2.0 Applications by Toby Segaran
Buy new: $26.39 / Used from: $10.46
A tremendous work on applying machine learning to common problems you'll run across when developing web apps. I was a tech reviewer for this book when Toby was finishing up his manuscript, and I was blown away from the very beginning at what a marvelous job Toby does of distilling what's historically been looked at hard to understand jargon into easily digestible, useful information