When I was tinkering with http://thredis.org http://thredis.org I spent a lot of time looking into SQLite internals, and it is by far the most polished, comment

When I was tinkering with http://thredis.org http://thredis.org I spent a lot of time looking into SQLite internals, and it is by far the most polished, comment-rich code base that I have ever seen, it reads like a fun book. Especially if you're interested in low-lever relational db stuff - how SQL statements are processed, data is stored, B-Trees, etc. It's better than any book out there (and such books tend to be very expensive).