This year DB2 celebrates its 30th anniversary. There is a flash book "IBM DB2 - The Past, Present, and Future: 30 Years of Superior Innovation" that you can download. The book was already distributed in printed version at the IOD Conference in Las Vegas last October. One chapter of the book gives a nice walk down memory lane to the beginnings of the relational data model and DB2.
To add to it, I wanted to point you to the HPI Genealogy of Relational Database Management Systems. It is a chart/poster that folks at the Hasso-Plattner-Institut put together. The poster shows many of the current and former database systems (including DB2, Oracle, Microsoft SQL Server, Sybase, MaxDB, MySQL, Netezza, Greenplum, ...), when they were released, how they were merged, rebranded, forked, acquired, etc.