I'm interested in general in all number theory, but especially in solving diophantine equations and expressions with $\bmod$. Someone can advise me few books or web sites where I could find more?
I have found the book: "Fundamentals of number theory" (William J.LeVeque, 1977). It's good?
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I have always found Burton's number theory book "just right" for a first course in elementary number theory; in addition, he has, in my opinion, an extremely well written, informative, book on the history of mathematics.
If you are a more advanced student, you may find Hardy and Wright more agreeable, but if not, Burton's text might be a good place to start. I have found his explanations to be comprehensive and clear.
According to Wikipedia, An Introduction to the Theory of Numbers
is a classic book in the field of number theory, by G. H. Hardy and E. M. Wright.
The fourth edition (1975 printing) is available on line.
The updated sixth edition was published in 2008.