George E. Andrews - Number Theory
Apostol - Introduction to Analytic Number Theory
These two books are great, I had not the time to finish to read them entirely yet. You'll find lot's of proof in them. I don't know if there is a solution manual for either of them but in Andrew's book you'll find a section "hint and answer" and there is a nice bibliography at the end of both books.
If you want to know more about mathematical proof in general you'll find a free good book on the website of the author: http://web.maths.unsw.edu.au/~jim/proofs.html (you have to scroll down a little bit to find the links to the pdf's...)
An other one is : Daniel J. Velleman - How To Prove It, published by Cambridge University Press.
Often they use theorems in Number theory as example in "Proof Theory". They show the different ways of proving things and depending of the problem you'll know what type of proof is adequate, how to make a strategy, etc...