CD roms are definately the best.
My driving instructor supplied me with one...else you can buy them from WHSmiths or other book shops etc.
Only took me a few goes on the computer, and I passed the theory first time.
Most of it is common sense.
It's very possible to study in 2 days, like someone said. You should get an up to date copy of the official DSA CD, I think I'm right in saying that they take the questions from there. I only got one question wrong after two days of study using that CD
It doesn't take any time at all to prepare for the theory test if you learn the stuff properly, you're moderately intelligent and you have a bit of common sense. I did mine before I'd ever driven, passed fairly comfortably first time, and spent no more than maybe 4 to 6 hours (spread over a few weeks) actively working towards it. Don't stress about it too much.