Do MySQL optimizations hold for MariaDB also?
Yes, all the basic optimizations in MySQL 5.5 apply to MariaDB 5.5. 

However, starting with MariaDB 10, a real fork, not all improvements in 5.6 are in MariaDB codebase, and alternatively, MariaDB has some exclusive features (Hash JOINS). For now, those are not too different, that may change in the future. For a book, where most of the optimizations are regarding basic indexing, it may not be very relevant. 

And you can always check the latest improvements online -blogs, wikis. This is a presentation I created for the newest MySQL 5.6 query optimizations techniques.

Have a look at this image for a visual representation (white zones are MariaDB-only). 5.6- and 5.7-only are not represented: