For number theory try this college textbook: A Friendly Introduction to Number Theory The first six chapters are online so you can see whether you like the app

For number theory try this college textbook: A Friendly Introduction to Number Theory The first six chapters are online so you can see whether you like the approach: http://www.math.brown.edu/~jhs/frintch1ch6.pdf http://www.math.brown.edu/~jhs/frintch1ch6.pdf