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:


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*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).

I would gift "Visual Complex Analysis" by Needham. It is a very beautiful book with a deep geometric intuition about complex numbers that is not typically covered in an undergraduate complex analysis course.
I would also gift Stillwell's "Roads to Infinity: The Mathematics of Truth and Proof", one of the most beautiful treatment of the infinity concept that I encountered and that does not stay at the undergraduate level.
