Readings in Database Systems, 5th Edition
I went through Sciore's book [0] while learning about relational database internals. As part of his book, the author also developed a minimal database system named SimpleDB [1] in Java. I can safely say that its source code is very easy to understand. Though the book is not free, SimpleDB is.[0]: http://eu.wiley.com/WileyCDA/WileyTitle/productCd-EHEP000711...[1]: http://www.cs.bc.edu/~sciore/simpledb/intro.html