Ebook: Elementary Theory of Numbers (Dover books on advanced mathematics)

The book covers main topics of elementary number theory. The book is very short (120 text pages) but not at cost of clarity: almost all theorems are proven in the text and many examples are given.

Not many problems have answer in the back, which is not good thing for self-studying.

The text does not require much mathematical background (I believe highschool is enough), and I can recommend the book to anyone interested in number theory. The book is very well worth its price. Buy this and if you still like number theory, buy one of those heavy books over $100 :-).