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).

Fifty challenging problems in probability. Here is a larger list. Hope it helps
