There are always great auctions going on around the country, including club auctions, philatelic exhibition auctions,
ebay, and so on.
Additionally, there are a couple of philatelic libraries that sell donated books at great prices.
The Northwest Philatelic Library always has books for sale, along with books that can be checked out. I'm having a hard time finding that list on the website, but you can contact Orlie Trier at nwpl@qwestoffice.net for a complete list of books for sale. Some are very reasonably priced!
http://www.nwpl.org/Additionally, you can contact Sergio Lugo at the Rocky Mountain Philatelic Society for a list of books they have for sale as well (rmpl@qwestoffice.net)
http://www.rmpldenver.com/start.doBrian