Building ASP.NET AJAX Controls - Further Reading
I intended to cite references in an earlier post so apologies if this seems like a bit of an afterthought - it certainly isn't. These references provided the bulk of the information for my series of posts on building ASP.NET AJAX controls. 
First and foremost is this book - "ASP.NET AJAX in Action" by Gallo, Barkol and Vavilala. It has a chapter on JavaScript for AJAX developers and a couple of chapters on controls (client and server). There is also a chapter on the AJAX Control Toolkit and in particular the API which I haven't even mentioned so far. My bad - if you're building ASP.NET AJAX Controls, take a look at the Toolkit and the SDK it provides.
Then of course there is https://www.asp.net/ajax where not only can you find links to download etc, but you can also find online documentation for ASP.NET AJAX 1.0 (the 3.5 documentation is on MSDN) and a selection of Video Tutorials. There are also some AJAX 1.0 tutorials online - these may be available in the MSDN docs for 3.5 but they don't jump out at me.
Those same video tutorials I mentioned above (and others) are also available at https://www.asp.net/learn/videos. Finally, you can find the ASP.NET AJAX Control Toolkit at https://www.codeplex.com/AtlasControlToolkit. That has lots of great controls, all the source code for you to examine / modify, a sample website showing the controls in action and that SDK I mentioned.