What do I need for A-level Maths?

If you don't mind the investment, a Casio FX 911ES PLUS will help tremendously in C2-4 exams (you can use it to check definite integrals, factorise, etc).

If you're on EdExcel, you'd definitely need the Pearson textbooks (whether you choose to download the Activebooks online, or purchase a used/new copy on Amazon/eBay/Waterstones).

I'd recommend getting familiar with the Physics and Maths Tutor website, ExamSolutions, as well as the Solomon/Gold past papers. Technically you can start teaching yourself some C1 and C2 material during the summer - I'd start with the algebra/quadratics/inequalities/calculus since they're fairly easy to self-teach.

EDIT - a laser printer with cheap toner/paper will be really important nearer exam time, you'd be surprised how many past papers are needed to achieve A and above grades at A-level.

Are you sure that calculator is allowed? lol. I did AS this year, and going into my S1 exam (nervous enough already) i was stopped because my calculator had the function to work out intergrals and algabraeic manipulation. i never noticed this, but at the start of ANY edexcel math (calculator paper) it says:
'Candidates may use any calculator allowed by the regulations of the Joint Council for Qualifications. Calculators must not have the facility for symbolic algebra manipulation or symbolic differentiation/integration, or have retrievable mathematical formulae stored in them.'

kinda sucked for me tbh cos I was given a REALLY OLD casio calculator which I just wasn't used to and didn't know how to use half the functions lol. My sister on the other hand (doing AQA) was allowed to take in a graphical calculator, which helped her a lot. point of me rambling is, check which exam board you're doing and buy your calc based on that