Which is the best book for Ajax beginners? I want a book that is simple and easy to understand....
@Alex, you missed SitePoint's AJAX book.
@minni, it's only available from SitePoint as a pdf. You could try contacting the books customer support or Shayne Tilley, there might be a few printed copys lurking about. Alterantivly you could try online auction sites, etc. I just had a look at Amazon UK's and they have some copys of the printed book in stock.
I recommend the titles: Foundations of AJAX (APress) and Bulletproof AJAX (FriendsOfED). AJAX isn't that simple of a subject as it's a mixture of different languages working together but they both handle the subject well. SitePoint (that I'm aware of) doesn't have an AJAX specific title on the shelves.