I used the ASA stuff. One book for the PPL, one for the IR, one for the CPL. And you get a CD in there with the computer exam software, IIRC.
Gleim has been recommended by many, but I never tried it.
If you actually fly for real already, the FAA theory is mostly easy enough. I just read through the books once, and did a load of computer exams. There is only about 20% dross, against some 90% in the JAA theory. The FAA PPL did have a fair few tricks in it though, like asking about weather over Michigan but not providing a map showing where Michigan is
Wouldn't work very well in the UK, would it?