John Stillwell, Elements of Number Theory (Springer 2002). This is by a masterly expositor, and is particularly approachable.
G.H. Hardy and E.M. Wright, An Introduction to the Theory of Numbers (OUP 1938, and still going strong with a 6th edition in 2008). Also aimed at beginning undergraduate mathematicians and pleasingly accessible.
Alan Baker, A Comprehensive Course in Number Theory (CUP 2012) is a nice recent textbook (and a lot shorter than its title would suggest, too).