As the title suggests this book is about Probability. It shows how to calculate probabilities and how to calculate density functions and other things. I especially like how it explains the symbols to the uninitiated.
Pitman has a bizarre aversion to calculus, so a lot of times he attempts to do these problems without integration, making things much more challenging. However, the book is a decent reference, very easy to read, and has an array of different problems to try out.
Honestly not as bad as everyone makes it out to be. The concepts are well-explained and intuitive but it lacks the depth of newer textbooks. It falls somewhere in between an introduction and an intermediate probability textbook.