I would go with a Monster book out of that list unless you have something in your campaign that may make one of the more specialized books fit better. You can never have too many critters!
I chose Libris Mortis because I use it more than any other book you have listed. Of course, if you don't use undead or necromancers, then it's of little use. If you do, it's darn near indispensable.
I love Frostburn. Provided your looking at arctic gaming. If you read Frostburn and didn't get at least 20 campaign/adventure ideas, your not trying hard enough...