Either of the first two books are a good place to start, then move on to the cubed book. I found it didn't have as much of the basic info in it.
IMHO, if you only want to buy one of the books? Check out one of the first two from your library and get familiar with choosing the fabric, finding the repeats, cutting instructions, etc. and then buy the OBW cubed if cubes interest you :D:D:D