I am currently studying my theory for the RA-Aus BAK and I hope to do the test soon but I plan to do my Radio and Passenger Endorsements at the same time. I have found a Ground Training Manual that I purchased from www.skysupply.com.au and it is called the RA-AUS Pilot Certificate - Ground Training Manual including the BAK, Air Legislation and Flight Radio Endorsement. It is written by Jan & Val Dyson-Holland. Each chapter you read has review questions and comes with an answer sheet. At the end there are then 4 practice exams for BAK, Pre-Solo Air Legislation, Pre-Certificate Air Legislation and Flight Radio Endorsement. The book also has a Study Guide and Progress Chart. The theory has been loosely based around the Jabiru J-160.
I have also found the Jim Davis book very good as well and well worth purchasing. As far as initial understanding of flight theory it is very well written and does not put you to sleep like a lot of training manuals do. Because Jim Davis refers to both Australia and South Africa, there are a lot of things that are more international standard and you need other theory to follow for Australian Recreational flying specifically.