There is the Jeremy Pratt series published by AFE or the Trevor Thom series published by Air Pilot Publishing. I have enjoyed using the Jeremy Pratt series - it came recommended with good descriptions and diagrams and is arguably more accessible. Others prefer the Trevor Thom ones which are perhaps more comprehensive and can be cross referenced more easily - but either set will work. Normally people go on the advice of their instructor but either will get you through the exams.
Buy them where ever is cheapest (I got mine on Amazon - some of them used). For Air Law get the most recent editions but for the others don't worry too much about which edition you get. For RT there is an excellent guide online so no need to get a separate book for it (I know its combined into one of the other Pratt books).
Enjoy!