I have not seen mention of Wladislaw Narkiewicz (forgive the lack of accents) and his books on the history of number theory, as well as Ribenboim's texts on certain Diophantine equations.  This could be for the lending portion of your library, for those who want to get their toe in the door without having much more than mathematical maturity and a stomach for notation.  I have not checked the bibliographies, but I imagine they would help as much as an index you made yourself for your library.

Gerhard "Not A Number Theorist, Yet" Paseman, 2014.04.17