Ask HN: People currently [Working] in compilers, what are your goto resources

LLVM - http://llvm.org/docs/

Specifically when learning and applying compiler development, advancing to more complex use-cases (code optimization, parallel compilation), programming language theory etc? Do you still use BNF Grammars with Parser Generators, or do you use Parsing combinators? Are you using a specified IR like LLVM's or if not why is that your choice? How do you deal with targetting multiple Architectures?