I recently read the paper `Animal Minds, Animal Morality` where the author thinks it should come down to the ability to experience 'experiences'. Its an interesting read.
How To Prove It is definitely the best (it's the only I read till now though, apart from online tutorials and course notes) book on how to learn on your own. Especially since it details the thought process on how to prove something, has around 20 questions per subsection with answers (especially this is great)!
Another book that I think is great is 'Getallen: van natuurlijk naar imaginair', but its in Dutch.
- Parallel graph rewriting - some contributions to its theory, its implementation and its application
The Clean programming language is build upon this, if I'm not mistaken.