We have a variety of textbooks because the government tinkers with the GCSE every couple of years. At the moment we are using 'GCSE Maths For Edexcel Specification B Modular'. It's not great. But in 2012 they'll all be replaced because Gove is going back to linear courses.
The best GCSE textbooks IMO were by Banks and Alcorn for Edexcel, back when we had higher, intermediate and foundation.