Sorry to read that you flunked the IREX. Don't take it to heart too much, it's a tough one. If you're acknowledging that your preparation was haphazard, then leave it at that and try again. I put two weeks almost full time into it before I felt that my sanity was taking a hit. That's when I concluded that I wouldn't be any more ready to take it.
In hindsight, knowing the AIP backwards and the front and back parts of ERSA proved the very helpful. There were probably 10 ridiculously straightforward, instant questions in my exam that solely tested your ability to navigate your documents/charts and extract the relevant information. I dedicated most of my time on alternate and fuel questions, most of which involved decoding the TAF/TTF and making sure that I was not missing the obvious. The other questions that took a bit of thinking were those on approach plates (MDA, DA, what to do if you don't see the runway environment, etc...).
I think that the exam is designed around the assumption that you should be able to get all those "easy" points (drift, track error, finding things in the documents, the expected emergency-type questions) plus around half of the more involved ones to get a pass.
Regarding Bob Tait's book: Other than the questions at the end and the brain teaser ADF/VOR chapter, I found the other parts to be somewhat lacking structure which made it harder for me to grasp the "spirit" of it all (I self-studied which probably didn't help for something as thick and ill-defined as IFR). I'm not saying it's a badly written book but for a casual VFR private pilot with close to no exposure to the IFR realm, I found the book a bit dry. If I have one regret though, it's delaying reading the actual AIP (RTFM, if you know what this means). Surprisingly, I found it to be quite concise and in fact rather to the point without much ado.
I also solved around 450 questions: those in Bob's book, 4 sample exams on Bob's site and 4 in Rob Avery's book.
Lastly, if I was to have a second regret, it's not heeding my IF instructor's advice when she suggested that I take a look at the sample IREX questions on CASA's website. Truly that was one of the poorer decisions I made in life because I had them open in one of the billion browser tabs and it wasn't until after taking the exam that I took a read. I was stunned to find 5-6 of the very same questions that were asked in the IREX. No drama but I could have saved precious time.