This is both a guide to developing with libvirt, and a useful reference document. It is a work in progress, contributed to by the members of the libvirt team and being authored by a professional author.
Contributors to this are VERY welcome, so if you'd like to get your name in this and demonstrate your virtualisation prowess, contributing solidly to the content here will do it. :)
The source is in a git repository:
git clone git://libvirt.org/libvirt-appdev-guide.git
Browsable here:
http://libvirt.org/git/?p=libvirt-appdev-guide.git;a=summary