There are books designed for the IB qualification
https://www.amazon.co.uk/Mathematics-Higher-Diploma-Option-Discrete/dp/1107666945/ref=sr_1_2?ie=UTF8&qid=1521592587&sr=8-2&keywords=a-level+discrete+maths
Or I read this and it was good
https://www.amazon.co.uk/Discrete-Mathematics-Norman-L-Biggs/dp/0198507178/ref=sr_1_3?ie=UTF8&qid=1521592587&sr=8-3&keywords=a-level+discrete+maths