LLVM Language Reference Manual
Defines the LLVM intermediate representation and the assembly form of the different nodes.
Introduction to the general layout of the LLVM sourcebase, important classes and APIs, and some tips & tricks.
CommandLine 2.0 Library Manual
Provides information on using the command line parsing library.
Details the LLVM coding standards and provides useful information on writing efficient C++ code.
Look here to see how to add instructions and intrinsics to LLVM.