I have just started studying for my PPL using the Trevor Thom books. I would recommend you also get yourself a copy of "The PPL Confuser" to prepare for the exams after you have done your studying. it also includes some info about the exams, that is time per exam and number of questions.
To answer your question, you can study one subject after the other and sit each exam whenever you are ready. You do have to have passed your aviation law exam before you can fly solo and you need to have passed the navigation and meteorology exams before you can fly solo cross country. Apart from this it is up to you in which order you sit the exams. Since you want to do all your ground exams before you start flying you could do them in what order suits you most.
The only other thing you should not forget, you need to have your medical before you can fly solo.
Have fun
GAF4139