I have did my A-level mathematics a long while ago and I did QM for about three to four months and got to Bosonic String theory now, but I would like to do some mathematics beside String theory. I do not know if differential equations are coverd in university but I did them to and I know how to solve them, so please do not suggest books on differential equations. My question is where to go from here. The fields that apeal to me are algebraic topology, number theory, polynomials, group theory. Any introductiory books on these fields or any other post A-level books are welcome. Thanks for any help.