If you want books for reference, I recommend the redbooks edition.
For begginers, there's a publisher, that I've read a time ago, with some good books, (http://www.murach.com/books/mainframe.htm
) Where get them for free, I don't know....
For DB2, first of all, a good book for SQL, Apress publisher has good ones...also it this DB2 book (http://www.flazx.com/ebook11157.php
) despite of I didn't read it :p
right now I'm reading DB2 for z/OS and OS/390: Development for Performance, Volume 1 (Volume Set)
by Gabrielle Wiorkowski,
maybe it's oriented for DBAs instead of for developers...but it has great chapters about physical design, and how it works...
I hope helping you.