Absolutely - Hardy and Wright's An introduction to the theory of numbers is excellent but definitely doesn't need to be restricted to postgrads - its also fine

Absolutely - Hardy and Wright's An introduction to the theory of numbers is excellent but definitely doesn't need to be restricted to postgrads - its also fine for undergraduate level number theory (indeed it was a recommended textbook when I was an undergrad). It is quite dense but at least personally I find that style of textbook much more useful than the American style enormous textbooks which takes a chapter to explain what could be said in a paragraph. You just need to know to expect it will take you quite a while to read each page.