Assuming you're doing a JAR PPL:
Book 1 is actually vital to the practical flight training. It contains all the flying exercises, and all the information you need to read before each lesson in order to get the most out of it.
Yes, there is more than 1 subject in each book. From memory, book 2 has both Air Law and Meteorology, and I think Performance is in the same book as the Aircraft Technical.
An old fashioned flight computer will be a good investment. It's the only calculator you're allowed in the exams and is excellent for Navigation related calculations as well as common conversions (meters to feet, litres to gallons etc). You don't need to go crazy spending £70 on a CRP-5. You can get old cardboard ones on ebay which are just as good.
I would also look to get either the PPL confuser, PPL perfecter, or PPL simplifier. All of these have loads of practice questions, many of which will be very similar wordings to the ones you will get in the real exam. A great help when some of the CAA exams are written in a very bad way making a misunderstanding very easy.
There's no specified order, but you will usually be encouraged to take Air Law first, usually before your first solo. From there, it's up to you to decide which exam is more relevant depending on how the practical flying is going (ie taking the nav exam as you begin the practical navigation exercises)