I would recommend a book called Very Thai. Lots of good pictures and explanations behind everyday things. I think it costs somewhere in the 900-1200 baht range.
Amazon.com: Very Thai: Books: Philip Cornwel-Smith,John Goss
For book buying, I agree that the airport is more expensive - I'm sure all the bookstores have the same kind of deal, but you can get 10% off every monday at Bookazine if you sign up to be a member. They have one in Jungeceylon. Buy a few books and it's almost like getting one for free!
Cheers, Robaht