Came across AJAX Patterns site and an O'Reilly book in the making. The most comprehensive and well written collection of AJAX tips and tricks I have come across so far. Besides that, there are a number of other aspects worth mentioning: it is being developed and edited using a WikiMedia software, the same software that powers Wikipedia, in a collaborative manner; the text is available under Creative Commons license; makes good use of Web technologies for intra- and inter-linking; and allows audience to participate in the content creation.
So far most authors and publishers have tried to publish content developed for print media on the web -- either PDF e-books or Safari style transformation (can't think of a better word) of the same content. Safari books are certainly much better for online perusal but neither take full advantage of the Web's potential. Ajax Patterns book is taking a different route -- it is not only getting created with help of Web technologies but is also architected for Web perusal. In fact I have a feeling that the author first got the idea of creating the content in this manner and the O'Relly book came as an afterthought. It would be interesting to see how this Web-centric content translates into printed material.