A perhaps oddball chocie, but for a guidebook I liked using:

THE MONUMENTS OF THE ANGKOR GROUP
by Maurice Glaize

(A translation from the 4th French edition).

This is a 1944 edition with nice black and white illustrations.
You may be able to find a free pdf downlaod without much trouble.

edit: go to this page:
http://www.theangkorguide.com/download.htm

right click, "save target as" on the link there.
7 mb