Sash Pachev has written a new guide on Understanding MySQL Internals for O’Reilly. I was one of the technical reviewers on this book and can tell you that it makes a good read if you want to understand more of what is going on under the hood in MySQL. There’s a lot to cover, for example the pluggable storage engine archtiecture, how queries are parsed and executed, and how individual storage engines work. The book is compact, but very readable, and worth a look if you any interest in how databases are programmed, or are thinking about building your own storage engine to plug into MySQL.