There is not enough, but a high level overview is here:
http://llvm.cs.uiuc.edu/docs/CodeGenerator.html#instselect
The best way is to learn by example, e.g. take a look at Target/PowerPC/PPC32ISelPattern.cpp.
The primary header files are in include/llvm/CodeGen/SelectionDAG*.h
Hope this helps, if you have any questions, llvmdev is a good place to go for help.
-Chris