Hellmuth"s books are AWFUL in my opinion. Really bad.
Sklansky"s books are so detailed that they"re deffinately a recommended read. The theory of poker is a really deep read. It can be a bit tough. It doesnt use hand examples that we"re used to with Harrington. It also draws examples from all the games, stud, razz, etc, so sometimes its a little tough to apply to hold em. But if you have the time to study it and really read through it then it will deffinately help your game.