The main source of information regarding LLVM is the LLVM website (www.llvm.org). This website hosts the main documentation (www.llvm.org/docs), including instructions on how to implement various parts of a LLVM backend.

In addition, the LLVM API documentation at www.llvm.org/doxygen contains useful information about available APIs and in many cases is the best source for information.

There is also a mailing list llvmdev@cs.uicu.edu and IRC channel #llvm on irc.oftc.net where questions about LLVM development can be asked.