Developer books
Below is a list of various books that might be useful in learning some basics needed for Qubes development.
A must-read about Xen internals:
The Definitive Guide to the Xen Hypervisor, by David Chisnall
Some good books about the Linux kernel:
Linux Kernel Development, by Robert Love
Linux Device Drivers, by Jonathan Corbet
Solid intro into Trusted Computing:
Dynamics of a Trusted Platform, by David Grawrock (original Intel architect for TXT)
Good book about GIT:
Pro Git, by Scott Chacon and Ben Straub (complete book available free online)
Useful books about Python:
Programming in Python 3, by Mark Summerfield
Rapid GUI Programming with Python and Qt, by Mark Summerfield (Although note that Qt is being replaced by GTK in Qubes code.)