I'm working in a program for teaching a group of students selected in a Olympiad competition. The program is aimed to acquaint the students with the diverse aspects of higher mathematics in a way which is attractive and giving them a good viewpoint of the subject. So I'm looking for books which can be used in the courses of this program. Note that I don't want a university textbook that contains the elementary parts of an abstract subject and not giving a general insight. As an example I can mention "Arnold's lectures in Abel's theorem".

Please help me with your advices for book titles, Thanks!