I had the same idea
nanocas, but I found it didn’t work very well in practice.
The problem is that the ATPLs cover such a vast array of subjects, and in such detail, that you would be swamped if you tried to read up on everything beforehand. An even if you picked only one subject, you would still need to focus on the bits that are relevant to (a) the syllabus and (b) the exam. That’s where the structured course comes in – when you start the course the study pack will guide you on which subjects to study and in what order.
Background reading will not do you any harm, but bear in mind that a lot of information needs to be learnt for the exam, and can safely be forgotten afterwards (e.g. the minimum size of registration letters on an airship, or the orbital altitude of GPS satellites).
Personally I would enjoy finishing the PPL, then through yourself into the ATPLs once you can focus on them without any other distractions.