My mum wants me to bin my school books when I finish my GCSE's

I guess it depends on why. Are they taking up too much space somewhere? If yes, and you don't need them to support A levels, then offer them to a freecycle/buy nothing group or a local charity that supports families in need. Actually putting them in the bin would be wasteful

Would other people use books that I have wrote in though?

I dont want to but she is forcing me to, what should I do??

Revision guides I am selling but im not sure what to do with exercise books

would someone else not use them or should i keep them forever?

i dont think anyone else would use them as all the same information is availiable online, i dont think theres much point in keeping them unless they're going to be really useful for A levels