Judging by the URL, this book was used for CMU's 15-151 / 21-128, which is a first-semester course for CS and math undergrads. Nowadays, the course uses [0].
[0] https://www.goodreads.com/book/show/445059.Mathematical_Thin...
[0] https://www.cs.yale.edu/homes/aspnes/classes/202/notes.pdf
That said, OP's book looks more conversational in tone, which I personally have a slight preference for.