Found this while looking for a good book on virtualization internals. It's a bit dated and limited to what Xen does, so I'll have to look for something newer, but it's a very good introduction to the actual workings and issues of virtualization. It also has a good amount of code to show how things are done.
A book with what I imagine must be a very limited audience. This is not particularly geared towards systems administrators supporting Xen, but geared instead for operating systems developers, Xen developers, and those seeking to use low-level Xen APIs. Regardless, this can be an essential guide for anyone, even systems administrators (with software development experience), looking to understand how Xen ticks.
This is a helpful reference to the inner workings of the Xen Hypervisor. It is easy to read and highly relevant for any software developer working with Xen.