Concrete Mathematics, by Graham, Knuth, and Patashnik. Not only is the subject matter interesting, and, AFAICT not presented all together as a coherent body anywhere else, there are literally notes in the margins that the authors say reflect the comments of students who took courses using the book while it was being developed. Even if you don't care about the subject matter per se, the comments are well worth the read.
Similarly, Proofs from THE BOOK, by Aigner and Ziegler presents some interesting subject matter (short, elegant, and instructive proofs) all in one volume that you'd have to comb through large amounts of mathematical literature to encounter otherwise. The results themselves should be mostly familiar to any grad student or advanced undergrad in mathematics, but the proofs are sure to amuse as well as enlighten. The one unfortunate thing about this book is that the last edition was published almost 25 years ago.
Similarly, Proofs from THE BOOK, by Aigner and Ziegler presents some interesting subject matter (short, elegant, and instructive proofs) all in one volume that you'd have to comb through large amounts of mathematical literature to encounter otherwise. The results themselves should be mostly familiar to any grad student or advanced undergrad in mathematics, but the proofs are sure to amuse as well as enlighten. The one unfortunate thing about this book is that the last edition was published almost 25 years ago.