Books on Number Theory :
Legendre, Gauss, Jacobi, Dirichelet-Dedekind, Kronecker, Minkowski, Hilbert , Hensel, Hecke, Weyl, Hasse, Artin, Artin-Tate, Lang, Weil, Iwasawa, Serre, Serre, Serre, ..., Tate, Cassels, Swinnerton-Dyer, Manin-Panchiskin, Neukirch, Borevich-Shafarevich, Fröhlich-Taylor, Kato-Kurokawa-Saito, Goldfeld...
If you still have some money, go in for the collected papers of arithmeticians
Gauss, Dirichelet, Eisenstein, Kummer, Riemann, Kronecker, Minkowski, Hilbert, Ramanujan, Takagi, Hasse, Hecke, Siegel, Artin, Weil, Iwasawa, Shafarevich, Serre, Manin, Tate (whenever they're published)
and conference proceedings, among them
Cassels-Fröhlich, Fröhlich, Cornell-Silverman, Borel-Casselmann, Szpiro, Jannsen-Kleiman-Serre, Cornell-Silverman-Stevens, Conrad-Rubin, Popescu-Rubin-Siverberg, Sarnak-Shahidi, Darmon-Ellwood-Hassett-Tschinkel, 2009 Clay Summer School
and arithmeticians' diaries
Gauss, Hasse,
and their correspondence
Hasse-Artin, Serre-Tate (worth the wait)
Finally, the history
Weil, Dieudonné
