what is the best book for Pre-Calculus?

You might want to have a look at the following (peruse them at your favorite online book store).

Precalculus Mathematics in a Nutshell: Geometry, Algebra, Trigonometry, George F. Simmons

Pre-calculus Demystified 2/E, Rhonda Huettenmueller

Some other food for thought.

You may also want to look at actual calculus books at your local university library and see some of the topics you might need.

You might want to get and learn to use a Computer Algebra System. You want to learn to explore mathematics and ask all sort of what-if questions and also learn about programming.

Regards

As a computer science student, you might also want to check out Knuth, Graham, and Patashnik Concrete Mathematics, which I consider to be absolutely indispensable for comp sci students. It's a very thorough book! Not necessarily recommended to replace Calculus, but provides the nuts and bolts of the basics in math that you'll want to master!

Major topics include:

Sums
Recurrences
Integer functions
Elementary number theory
Binomial coefficients
Generating functions
Discrete probability
Asymptotic methods

And more...

"This book introduces the mathematics that supports advanced computer programming and the analysis of algorithms. The primary aim of its well-known authors is to provide a solid and relevant base of mathematical skills - the skills needed to solve complex problems, to evaluate horrendous sums, and to discover subtle patterns in data. It is an indispensable text and reference not only for computer scientists - the authors themselves rely heavily on it! - but for serious users of mathematics in virtually every discipline."

"The book includes more than 500 exercises, divided into six categories. Complete answers are provided for all exercises, except research problems, making the book particularly valuable for self-study."