If you're already familiar with basic web concepts and Java then you should be able to delve right into this book. Otherwise, may I humbly suggest our first book, "Foundations of Ajax," as a good Ajax primer:
Ryan Asleson<br />Co-Author, <a href="http://www.amazon.com/Foundations-Ajax-Foundation-Ryan-Asleson/dp/1590595823/sr=1-1/qid=1158926537/ref=pd_bbs_1/002-7412154-2924860?ie=UTF8&s=books" target="_blank" rel="nofollow">Foundations of Ajax</a> and <a href="http://www.amazon.com/Pro-Ajax-Java-Frameworks/dp/1590596773/ref=sr_11_1/002-7412154-2924860?ie=UTF8" target="_blank" rel="nofollow">Pro Ajax and Java Frameworks</a><br /><a href="http://taconite.sf.net" target="_blank" rel="nofollow">Taconite</a> Lead Developer
Tony, you can download and read chapter one at Apress's homepage. The chapter is called "What Is Ajax?", and is only 25 pages, so don't expect much from it. You can also try "Head Rush Ajax" for a brain friendly introduction.