I personally wouldn't use other countries texts for IFR (or Air Law) as there are quite a few things that Australia has (or doesn't have) that differs from ICAO and if you are using American texts, ICAO and FAA differ a bit as well.
I like Bob Tait as a starting point, other people hate him. Search the forum for everyones opinions, it has been discussed many times and make up your mind which textbook to use.
I found IREX was essentially 2 things, Air Law and instrument reading. So know how to read and use VORs and NDBs and then know the IFR Air Law (eg; ENR 1.1/1.5 + a few other areas in the CAOs, CARs and AIP). Oh plus the BOM weather manual, which you will get 1 or 2 questions from!
If you are self studying IREX I would at least book in an hour or two with an instructor every few weeks to go over questions that will come up as some of the regs are a bit 'fiddly' when you first read them.
Obviously this is my personal opinion and may not work for you!