I almost always have reference documentation open on my desktop: browser windows with API docs, man pages in terminal windows, Acrobat files. It’s too hard to remember every detail about infrequently used calls and more obscure language syntax; I’ve got better things to do with my brain. It’s strange, I’ve always preferred learning by examples + reference docs, rather than by hand-holding tutorials or “how to” books. I like the anal-retentiveness of official standards. =)
AJAX certainly requires familiarity with a lot of different specs. It was difficult initially to know where to look for something, so hopefully this list will help someone out there who learns the same way I do.
Document Object Model (DOM) Level 2 Core – This spec gives you info about the fundamentals of the DOM. This is the “base” upon which the HTML DOM is built, so it’s very useful for figuring out how to traverse the document hierarchy and manipulate it in basic ways.
Document Object Model (DOM) Level 2 HTML – This spec describes the HTML-specific DOM. This is the “meat” of building any highly dynamic, interactive application, since you’ll certainly need to know how to manipulate HTML elements.
Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) – I look here not just for info about stylesheets, but for an understanding of the model for how document elements are (supposed to be) laid out. Go here when something looks misaligned or off-kilter, or when trying to programmatically control aspects of visual layout.
XMLHttpRequest Object – Without this object, it ain’t AJAX. This baby is how the browser performs asynchronous data requests and conveniently makes available XML responses as Document objects. Neato!
Prototype.js API – This is a pretty amazing library that extends the DOM, providing all sorts of extras for working with document elements and ensuring cross-browser compatibility. It’s not a library for fancy effects or pre-built interface widgets; rather, it makes up for the convenience deficiencies in some of the DOM specs.
Boy, that’s a lot of docs. Right now, I’m still looking for good info on support and compatibility for various versions of DOM and CSS in various browsers.