I want to get a book for my maths teachers. They are both very able: one has a PHD in maths from Cambridge University in the UK, and the other was at the IMO. Because of this, I am unsure of which books are the right difficulty, as the books I do are much easier (e.g. Calculus by Spivak)
One has a particular interest in Analysis, so any recommendation on that would be appreciated. The other specialised in Combinatorics at university, so something along those lines would probably be best. However, any recommendations of books at the right difficulty would be great.
Thank you!!
Edit: having read the comments, I will probably ask them if they have the book before buying it.