Hi Martin,
If you're registered on an approved ATPL Distance Learning or Residential course (you *have* to be doing one or the other) then you will be supplied their own notes.
These normally come in the form of big, thick, A4 size folders with colour pages and lots of text to read through. They'll also have assignments and tests to complete and send back to them for marking.
Assuming you already have these then some suggested reading materials:
Ground School for Pilots volumes 1 to 4
These are written by Underdown and Cockburn and cover Radio Aids, Navigation, Flight Planning and Meteorology.
Also the four yellow books Mike Burton has produced for the technical subjects are great.
If you're on a course other than Oxford then you could always go to Transair and buy the Oxford manuals at around £600 for the lot.
Best wishes,
Charlie Zulu.