To be honest I would use whatever you can get. People take in and learn information in different ways so what is good for one person may not be the best for another.
All the manuals are available second hand quite cheaply. I used the BGS books which I thought were very good but then had the Phil Croucher book and bought a set of OAA manuals used too. One of the best resources now is YouTube and if you are struggling in a subject there will be videos explaining it on there. The OAA CBTs are on there with different narration and these are good.
Use whatever you can get to get a good understanding of all the subjects.