TITLE: Undergraduate/High-School-Olympiad Level Introductory Number Theory Books For Self-Learning
QUESTION [14 upvotes]: I don't know whether the books metioned in Best ever book on Number Theory are beyond undergraduate/high-school-olympiad level.
Please recommend your favourite.

REPLY [3 votes]: Davenport's The Higher Arithmetic was my first number theory book. I think its very accessible to a high school student or beginning undergraduate student. It's quite short and very quickly readable.
If you find this treatment too informal, Niven and Zuckerman's an Introduction to the Theory of Numbers is a standard text that I think is a very well written undergraduate text, but this has already been mentioned.