I feel like TFA makes a very bad case for it (they use structs? only 8 functions have many parameters? your commercial tool can query this kind of thing with SQL?), but regardless, SQLite is a very good codebase to delve into as a seasoned programmer wanting to improve even more.

What worked for me was diving in with a specific question (I think it was "I wonder how indexes are represented on disk?") until I found the answer, picking up a lot on the way.

Not an easy codebase to read, but full of good architecture, good ideas and best practices.