Reading list
There are very many books on elementary number theory. The best book
to learn the material is the one you find the best. Here is a
somewhat random and unsorted list:
An introduction to the Theory of Numbers: Hardy and Wright
An Introduction to the Theory of Numbers: Niven (and other authors)
A concise introduction to the theory of numbers: Baker
Elementary Number Theory: Burton
A Friendly Introduction to Number Theory: Silverman