There are lots of good books as well, of course. Too many to list.
A search on Amazon.com gives 3980 books when searching for "DB2".
Here's the top-10 from them:
Understanding DB2: Learning Visually with Examples by Raul F. Chong, Clara Liu, Sylvia F. Qi, and Dwaine R. Snow (Hardcover - Jan 25, 2005)
DB2 Developer's Guide (5th Edition) by Craig S. Mullins (Paperback - May 11, 2004)
DB2 UDB v8 Handbook for Windows and UNIX/Linux by Philip K. Gunning (Paperback - Aug 6, 2003)
DB2 UDB V8.1 Certification Exam 700 Study Guide by Roger E. Sanders (Paperback - Sep 17, 2003)
DB2 SQL PL: Essential Guide for DB2 UDB on Linux, UNIX, Windows, i5/OS, and z/OS (2nd Edition) by Zamil Janmohamed, Clara Liu, Drew Bradstock, and Raul F. Chong (Hardcover - Nov 15, 2004)
DB2 for the COBOL Programmer, Part 1, 2nd Ed. by Curtis Garvin and Steve Eckols (Paperback - Jan 1998)
DB2 for z/OS Version 8 DBA Certification Guide (IBM Press Series--Information Management) by Susan Lawson (Hardcover - Oct 28, 2004)
DB2 for the COBOL Programmer, Part 2, 2nd Ed. by Curtis Garvin, Anne Prince, and Steve Eckols (Paperback - Oct 1999)
Advanced DBA Certification Guide and Reference for DB2 UDB v8 for Linux, Unix and Windows by Dwaine R. Snow, Thomas Xuan Phan, and Dwaine Snow (Paperback - Jul 7, 2003)
DB2 UDB V8.1 Certification Exams 701 and 706 Study Guide by Roger E. Sanders (Paperback - Dec 12, 2003)
DB2 for z/OS and OS/390 Development for Performance (Volume 1) by Gabrielle Wiorkowski (Hardcover - Feb 2002)