[gtrubetskoy]

I have to say I learned more about databases from just studying SQLite code than any book on the subject. I've bought a bunch of books on DB's, some very expensive ones, but I wish someone pointed me to SQLite source early on.

To internalize it better I invented a "project" for myself - http://thredis.org/ which was (and is, but I'm not maintaining it) a Redis/SQLite hybrid. It was fun to hack on.

Another invaluable source of DB internals information is PostgreSQL. Both projects have amazingly well written and detailed comments.