1. Handbook of Mathematics, by I. N. Bronshtein, K. A. Semendyayev (Contributor), K. A. Hirsch (Editor), Hardcover, 973 pages 3rd edition (1997), Springer; ISBN: 354062130X

This is not a textbook, but a guide and handbook for mathematics. It treats just about every subject you are likely to have had in an undergraduate class in mathematics for engineers, but with less explanations and no homework. In addition it contains enough material about many branches of mathematics so that you will at least be able to find out what area of mathematics you need to solve a problem, and gives you at least some information about some basic and even advanced techniques in the field.
You should not let yourself be deterred by the fact that it has some clearly outdated information about computers. If you have sold your calculus textbook, and sometimes think that was a bad idea, it is worth considering buying this book, which costs about $ 60. I know that twenty years ago (almost) every engineering student in Germany owned the German translation of this book. If you want to take a look at it, you can do so in my office.

2. Numerical Recipes in C: The Art of Scientific Computing, by William H. Press, Saul A. Teukolsky, William T. Vetterling, Brian P. Flannery, Hardcover, 994 pages, 2nd ed. (1992), The Press Syndicate of the University of Cambridge, ISBN: 0521431085

This book addresses the methods available to numerically solve a great variety of different problems, with explanations about how and why these methods work and what can go wrong when one is using them. It also contains ready-made code in C, which you can also get on a CD, for these problems. Versions in other programming languages are available.