Fun but serious mathematics books to gift advanced undergraduates.

I am looking for fun, interesting mathematics textbooks which would make good studious holiday gifts for advanced mathematics undergraduates or beginning graduate students. They should be serious but also readable. In particular, I am looking for readable books on more obscure topics not covered in a standard undergraduate curriculum which students may not have previously heard of or thought to study. Some examples of suggestions I've liked so far: On Numbers and Games, by John Conway. Groups, Graphs and Trees: An Introduction to the Geometry of Infinite Groups, by John Meier. Ramsey Theory on the Integers, by Bruce Landman. I am not looking for pop math books, Gödel, Escher, Bach, or anything of that nature. I am also not looking for books on 'core' subjects unless the content is restricted to a subdiscipline which is not commonly studied by undergrads (e.g., Finite Group Theory by Isaacs would be good, but Abstract Algebra by Dummit and Foote would not).