The Red Book [1] (recently updated) is a good overview and has lots of useful references. Some might be too high-level, but this paper (really a short book!) is a good start at what makes up a database: "Architecture of a Database System"[2].
Keep in mind that the scope of a "database" can be huge, so you might want to start really small, e.g., writing a basic key-value store that can support things concurrent reads & write, isolation, etc. It all depends on what interests you.
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[2] http://www.electricmonk.nl/data/freeitebooks/fntdb07-archite...