With the groundschool I am with you receive a complete set of manuals covering several subjects, which I assume is the same for the other schools.
It may be better to concentrate on your PPL first, get that under your belt, and then move onto ATPL's.
I can understand your eagerness to get on and have a preview of what is to come, but buying just one manual may be wasting your money. Why not look at some of the pilot supply shops and buy a good book or video which offers advice on how to make it in the industry, maybe one offering hints and tips on interviews, etc.
If you are dead set on an ATPL manual then you could buy one of the Jepp manuals without signing up to a school.