The Document Object Model and XHTML
Cascading Style Sheets
Events and Asynchronous Requests
Potential Problems for Ajax
Another potential problem has to do with indexing by search engines. Content that is generated dynamically by Ajax interacting with the user will not be visible to search engine robots.
The Ajax book I recommend is "Ajax in Action" by Crane, Pascarello and James, published by Manning, ISBN 1-932394-61-3.
For a compact summary of the state of ECMAscript standard usage in various environments consult the Wikipedia entry: http://en.wikipedia.org/wiki/ECMAScript
For the current state of DOM standardization see: http://www.w3.org/DOM/
For the current state of CSS standardization see: http://www.w3.org/Style/CSS/
See SearchWebServices' Ajax Learning Guide collection of Ajax resources