* Functions and Graphs by Gelfand et al. - A small but great book to develop intuition.
* Who is Fourier? A Mathematical Adventure - A great "manga type" book to build important concepts from first principles
* Concepts of Modern Mathematics by Ian Stewart - A nice overview in simple language.
* Mathematics: Its Content, Methods and Meaning by Kolmogorov et al. - A broad but concise presentation of a lot of mathematics.
* Methods of Mathematics Applied to Calculus, Probability, and Statistics by Richard Hamming - A very good applied maths book. All of Hamming's books are recommended.
There are of course plenty more but the above should be good for understanding.