Hey nanocas! If you are reading ahead on the ATPL I would recomend you start with "Principles of Flight". Performance is based on that subject and alot of stuf from PoF show up in other subjects as well. So dont start with Performance, that will only confuse you.
I would also recomend that you read the easier subjects like "VFR/IFR Com", "Mass and Balance" and "Human Performance".
When time comes get a good qestion bank, not Aerosolutions Abacus (it was good for practising but very few official questions), that will save a lot of time.
I am sorry to say that the time when you study ATPL will not be the best of your life, but on the other hand when you finnish things will look alot brighter. Belive me, I know, I just finnished my exams this friday.
regards
D.