You have a nice starting point in the free book:
http://www.lulu.com/morphxit
It is from 2006 but the basics works those days. When you are familiar with the environment I can recommend the "Inside Dynamics AX" books (versions 4 or 2009) from Microsoft Press. Those are impressive books from the AX development staff directly.
Microsoft Development Center has lots of information (videos!) for developers:
http://msdn.microsoft.com/en-us/dynamics/ax/default.aspx
If you know C# or Java you are not going to experience many problems with the language, it is all to be fluent in the MorphX environment, that onesty, is way easy in it's basis, but you have to work and fight with lots and lots of standard classes. That is, for my experience, the most dificult part of AX learning. Easy part, is that almost every one of those classes are docummented somewhere.
If you have PartnerSource access, in the learning & certification menu you have access to the materials of the official courses. There are 4 progressive courses of developing and those are a very nice starting point if you have access to it.
http://www.microsoft.com/learning/en/us/certification/dynamics-ax-professional.aspx#dev2009
Here you can find all the certification Exams, and inside each of them, you can find the course code. With this codes you can searh inside PartnerSource to find the appropiate materials of the courses.