If you DM, The Monsters know What They're Doing.

If you are a player, Live to Tell the Tale: Combat Tactics for Player Characters.

Personally I got more out of the first first. A lot of the stuff in the player focused book boils down to "understand your character's race/class options work and what can trigger them." Sitting on the other side of the screen makes you think that way more often anyway. Not unhappy I got it, just don't reference it as much as the first.

Other things you might not think about are the Illustrated Dictionary of Historic Architecture and the The Storyteller's Thesaurus for game prep when the words you want to use are not present in your brain.