I would echo OP's suggestion of learning approximation problems for getting better at math.

There is a course on MIT OCW by Prof Sanjoy Mahajan called Street Fighting Mathematics: https://ocw.mit.edu/courses/18-098-street-fighting-mathemati....

It uses a book of the same name. You can find more about it here: http://streetfightingmath.com/index.html

There is also Concrete Mathematics by Don Knuth et al. You can find it for free as a pdf if you google properly. It is a mixture of discrete and continuous math.

Also Polya's How to Solve It.