The internals of PostgreSQL
A really cool paper to read is The Design of POSTGRES [1] by Stonebraker et al. It's dated by its mentions of things like POSTQUEL, but it's still really interesting to read about the early design of seminal features like the extensible ADT system, from a time when it was still innovative.1: http://db.cs.berkeley.edu/papers/ERL-M85-95.pdf