Besides the classical textbooks (Ertl/Küppers or Hüfner) I always recommend taking a look at available manuals. Especially the "theory of operation" part in the Omicron manual is extremely well-written and very close to the actual application:
http://uhv.cheme.cmu.edu/manuals/M470101.pdf
I would recommend you start with that; if you want to dig deeper into the theory behind it, I suggest Ertl/Küppers, if you need more application or evaluation details, the best place to go to is the Casa XPS manual:
the XPS technique is extremely fascinating and rich in nuances; the book that I (.... and many colleagues) consider fundamental in the sector, and which I strongly recommend is:
SURFACE ANALYSIS by Auger and X-Ray Photolelectron Spectroscopy, Edited By David Briggs and John T. Grant Published by IM Publications and Surface Spectra Limited (2013), ISBN 1901019047
I finish by writing what the eminent physicist, Wolfgang Pauli, used to say, “God made the bulk; the surface was invented by the devil. "