A Friendly Introduction to Number Theory, 4th ed. by Joseph Silverman, 2012
Elementary Number Theory, 7th ed. by David M. Burton, 2011
Number, Shape, & Symmetry: An Introduction to Number Theory, Geometry, and Group Theory by Diane L. Herrmann and Paul Sally, 2012
Number Theory: A Historical Approach by John J. Watkins, 2013
Number Theory - Structures, Examples, and Problems by Titu Andreescu & Dorin Andrica, 2009
