What books are you going for?
Amazon is the cheapest for the Jeremy Pratt series, but possibly not for the Trevor Thom series.
There are always PPL books being sold off on ebay so that would be well worth a look, although I have seen items sold on ebay for close to or even more than you can get them brand new from a retailer!
Don't forget (or rely on) the PPL Confuser - the one with the green cover is the latest edition.
All the best.
9mm