After the filesystem and simple library packages like dbm, Berkeley DB was the original luxury embedded database widely used by applications as their core database engine. NoSQL before NoSQL was cool. The hidden secret making complex applications sing. If you want to dispense with all the network overhead of a server based system, it's still a a good choice.
There's a great writeup for the architecture behind Berkeley DB in the book The Architecture of Open Source Applications. If you want to understand more about how a database works or if you are pondering how to build your own, it's rich in detail, explanations, and lessons. Here's the Berkeley DB chapter from the book. It covers topics like: Architectural Overview; The Access Methods: Btree, Hash, Recno, Queue; The Library Interface Layer; The Buffer Manager: Mpool; Write-ahead Logging; The Lock Manager: Lock; The Log Manager: Log; The Transaction Manager: Txn.