Bookshelf (updated)
Archaeology Related Texts:
Digital Archaeology by Thomas L Evans and Patrick Daly
Spatial Technology and Archaeology by David Wheatley and Mark Gillings
This is one of those texts that anyone who is serious about doing good quality geospatial analysis with archaeological data should read.
Landscape Archaeology and GIS by Henry Chapman
Another good read on applying geospatial analysis and archaeology, with an emphasis on landscapes rather than specific sites
General GIS:
Open Source GIS: A GRASS GIS Approach by Markus Neteler and Helena Mitasova
This was incredibly hard to get hold of for a while, but the third edition is now out, and it’s a must if you want to get into using the grand-daddy of GIS packages.
(NEW!) Desktop GIS: Mapping the Planet with Open Source Tools by Gary Sherman
Everything you ever wanted to do with Open Source desktop GIS, with a focus on Quantum GIS. Not just an introduction, but a toolkit, taking on more advanced options such as scripting and command-line fu.
Web-Based GIS:
Web Mapping Illustrated by Tyler Mitchell
For an outline of the whole area of web-based mapping, this is hard to beat. Amongst other things, it gives you the clearest set of instructions for installing PostGres/PostGIS that I’ve seen, and a very useful glossary of MapServer commands.
PostgreSQL:
Beginning Databases with PostgreSQL: From Novice to Professional, Second Edition by Neil Matthew
A great introduction and reference to working with PostgreSQL- I use this every day near enough!

