Here are some interesting books to read with calcmod :
https://itunes.apple.com/fr/book/number-theory-rsa-cryptography/id1441161094?mt=11 
https://itunes.apple.com/fr/book/number-theory-rsa-cryptography/id1441215859?mt=11
Available on iBooks store, from Pascal Cardin. The student version (violet) is FREE !