The document is the key, but will only go as far as being able to get you through the first half of stage one. They mark the NATS test and if you pass you get to spatial awareness and diagramming. If you don't pass the NATS test you have to go home. If you fail spatial awareness or diagramming you are no better off than if you failed the NATS test originally. Learn the documents, learn the plane information, and practice!
You might get a couple of general knowledge questions which weren't in the booklet. They are the sort of questions which, if you have a real vested interest in the aviation industry, you ought to know the answer to. So read around a bit as well - the more you know about ATC and ATS the better off you'll be.