Yep. I was going to say Redis and SQLite. Both are really well commented. They almost read like a manual.

Although still in beta, I'd like to add BearSSL to the mix of well written and documented C libraries. In particular compared to the OpenSSL "documentation". It's also nice to see an TLS implementation without any memory allocations at all.

> Redis Source Code is understandable even without deep knowledge of C

Came here to say exactly this - Redis is very cleanly written.