**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, ...

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

...