If you think you are roughly at A-level, get a quick refresher:

* Basic Mathematics by Serge Lang

* Calculus Made Easy by Silvanus Thompson

Then try a DIY bootcamp in the spirit of Harvard Math 55. If you are interested in continuous math:

* Vector Calculus by Hubbard & Hubbard

If you are interested in discrete mathematics and computation:

* Logic in Computer Science by Huth & Ryan