First Steps in Number Theory: A Primer on Divisibility

This book deals with tests of divisibility and the rich theory behind them. Along the way, the reader will study a subject called Number Theory. To study this book, all that is required is familiarity with elementary arithmetic and algebra (addition and subtraction of algebraic expressions, the laws of exponents, the idea of prime factorization of an integer, the notion of relative primeness of two integers, etc.); in short, material which would normally  be covered in grades 7–9 in most countries. Plenty of exercises are scattered throughout the book, with solutions at the end.