If you expect to get A or A* in maths then take the summer off - if you have a decent teacher then S1 and C1 will be easy. If you expect to get a B then the CGP Headstart book is useful to cover some of the higher stuff you may not have mastered.
Heinemann C1 and C2 text books were OK though some of the Exam level questions were taken from old Pure papers and are way harder than anything that comes up now, I found S1 too wordy and very difficult to self study from. They have reworked their text books for AS next year so don't know what the new versions are like.