
Architecture of a Database System presents an architectural discussion of DBMS design principles, including process models, parallel architecture, storage system design, transaction system implementation, query processor and optimizer architectures, and typical shared components and utilities.
[1] http://db.cs.berkeley.edu/papers/fntdb07-architecture.pdf
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...
[1] http://db.cs.berkeley.edu/papers/fntdb07-architecture.pdf