Which the best book you've read till date in computer science or maths?

Depending on what you're looking for, Journey Through Genius by Willian Dunham is a very interesting history of mathematics that goes through many famous results in math, from Euclid to Cantor. It's accessible but still mathematical.

Code: The Hidden Language of Computer Hardware and Software by Charles Petzold.

Structure and Interpretation of Computer Programs (SICP)

To mock a mockingbird, Raymond Smullyan.