you might find the following links useful. some have books listed, you can find reviews of them to decide if you think they'll be a good fit. others are just helpful sites or advice.
EasyMaths - Free GCSE Maths tutorials, Sample Maths Exam papers, Books, Puzzles, Magic all helping to make learning maths easy!
What are the best GCSE Revision Sites/Books? - Yahoo! UK & Ireland Answers
GCSE Maths Books
maybe e^(i*pi) will have some recommendations...