LLVM has <b>extensive</b> <a href="/docs/">documentation</a> 
describing the high-level aspects of the compiler system in good 
detail.  LLVM also includes <a href="/doxygen/">doxygen</a> and <a 
href="/cvsweb/cvsweb.cgi/llvm">CVSWeb</a> documentation for the 
low-level aspects and individual API references.
LLVM is one of the best-documented compilers available.