What IF? - Randall Munroe
What if? is an incredible book. It is hilariously funny. I read the whole thing in a few days, engrossed in it throughout. With calculations, big numbers and ...
Well, it is just incredible. The author also has a webpage (with everything that is in the book available for free):
Here is a question from the book (click on the text to see the answer):
What would happen if you tried to hit a baseball pitched at 90% the speed of light?