For what it's worth....
I've recently passed RT. I don't think you can learn it from a book (although you may get through the written). Unless you spend LOTS of time on it (i.e. months and months at an hour a day) I don't think you'll get through the exam without 1 on 1 tuition.
Also whilst the radio scanner may help I suspect you'll end up picking up lots of bad habits "G-RDIO with you" etc which will be even harder to remove and may make things more difficult in the longer term.
Read the book (I recommend Jeremy Pratt's PPL2 "Communications"), then get the 1:1 tuition when you've done some flying, then get the exam done.
CAP 413 is an almighty document to wade through and only about 50% of it is actually tested, so you are wasting your time using that solely imho.
PS If you really want a radio scanner, but don't want to buy one then I'll send you mine for free. I used it once or twice but didn't find it in anyway useful to get through the practical exam.