David (Truman State University, Kirksville, Missouri, USA) Garth & Anthony (Truman State University, Kirksville, Missouri, USA) Vazzana 
Introduction to Number Theory [PDF ebook] 

Introduction to Number Theory is a classroom-tested, student-friendly text that covers a diverse array of number theory topics, from the ancient Euclidean algorithm for finding the greatest common divisor of two integers to recent developments such as cryptography, the theory of elliptic curves, and the negative solution of Hilbert”s tenth problem.