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
Serre
Tate (whenever they're published)
and conference proceedings, among them
Cassels-Fröhlich
Cornell-Silverman
Borel-Casselmann
Jannsen-Kleiman-Serre
Cornell-Silverman-Stevens
Conrad-Rubin
Popescu-Rubin-Siverberg
Sarnak-Shahidi
...