You'll probably need to know a little about compilers and/or type systems to be effective working on mypy internals. In particular, understanding ASTs, parsers and symbol tables would be useful, but code generation and various optimizations less so, unless you are working on mypyc.

Introductory texts:

• The Dragon Book
• Modern Compiler Implementation in Java/C (Appel)

More advanced books:

• Types and Programming Languages (Pierce)
  • This is heavier on theory

• Parsing (Wikipedia)
• Abstract syntax tree (Wikipedia)
• Symbol table (Wikipedia)
• PEP 484 - Type Hints
• PEP 483 - The Theory of Type Hints