Self study is the way to go. However from my experience the Bob Tait books will get you to pass the test however in the last 2 tests i recently did for my cpl (Met and Gen Knowledge) there were a couple of questions in the actual CASA exam that was not covered in the Bobs books. One question related to isotachs and isotherms and the other was an in depth question on fire systems. Since i have both Bobs and also aviation theory centre texts, the ATC books even though they go into a lot more depth had the answers to the above questions where Bobs fell short on those two question.