by Stephen Pressfield. It's a novel, but is incredibly researched and goes into more depth about Spartan culture and the Battle of Thermopylae itself.

Check it out:
http://www.amazon.com/Gates-Fire-Novel-Battle-Thermopylae/dp/055338368X/ref=pd_bbs_sr_1/002-2883001-3768005?ie=UTF8&s=books&qid=1173584004&sr=8-1

I actually wish they would've adapted this story into a screenplay.