You can find a book via google or other file sharing sites called

Elementary Number Theory by David M Burton

It has a lot of examples to practice, it covers Fermat's, Euler, Chinese Remainders, Least Residues, etc.

For the groups, fields and other math from the beginning, this book is good too

A first course in Abstract Algebra by John B. Fraleigh


These are good books that the math dept teaches these topics from.