ms, your DFR should have given you some info on the aptitude test otherwise it can be found on the defencejobs.gov.au website. This example is pretty much what you can expect for JOES, so if you can do them without a problem then you should be fine.
Other than that, just brush up on your math and try to find some of those career based aptitude tests, i.e. the sort you do when applying for a supermarket job online
. Read the posts above and other threads that relate to the aptitude testing, they will give you a good idea as to what you can expect.
Most importantly make sure you present yourself as officer material on the day. ie. dress smart, be confident and know what you want. Also, if you haven't started researching the job then do it now and know it like the back of your hand. When you book your JOES with a defence recruiter they will pretty much guide you on what you need to know and answer any questions you might have, or refer you to someone who does know the answer(s).
If you are looking for specific books on aviation aptitude testing, well there are the american guides for their military aviation testing, but there are none (that I know of) regarding the Australian Defence Force. Best bet is to sift through the internet and try to find some general aptitude and IQ tests or visit a library/bookshop to see what they have on the topic.
Cheers