Personally I would plump for a batch of vids or dvd's - much easier and often more interesting to learn from than the dry text of any manual. Most schools will provide their own manuals anyway in each subject, and groundschool will follow these. Best not to confuse matters by using one set of manuals to prep from and then having to study properly from another. Could prove a major headache and a waste of money.
BWould also suggest avoiding anything specifically aimed at JAA syllabus and concentrate on something more general for building up a wider understanding of the subject. Agree met would be a good choice as it is a fascinating subject - all the techy stuff is a real drag (yes, I know we have to know it but it still bores me rigid). Alternatively, some of the flight deck vids might be worth a watch just to see how all this knowledge and training is put to use after licence issue.