It may very well be the case that looking at the internals of some database other would provide a different set of lessons.
IMHO, MySQL is especially ugly due to an attempt to insert an abstract interface around its storage engines. IIRC that interface is leaky, and its documentation is pretty spotty. Also, the InnoDB storage engine has a lot of complexity that seemingly duplicates what's provided on the storage-engine-agnostic side of that interface.
One of the big lessons I took away from studying MySQL's internals, and partially confirmed by the "internals" book, was this: Be careful when designing a plugible storage-engines framework for a DBMS; MySQL has some examples of what can go wrong.
That's something you're unlikely to find mentioned in a generic DBMS-implementation book, but is very good to know about regardless.