Bob Tait for IREX. Make sure you subscribe to his online practice exams, too.
Definitely agree with this one.
Did mine about 5 years ago with nothing more than Bob Tait's book and got around 95%. His online practice exams are definitely worth it as well.
Also suggest you might pick up the ATC instrument rating theory book as well. Bob Tait's book is perfect if you just want to pass the exam (and he has a very good simplistic way of explaining the theory), reading the ATC book afterwards gave me a better overall picture of everything before jumping into training for the instrument rating.