"The Department of Mathematics at the University of Illinois at Urbana-Champaign has long been known for the strength of its program in number theory. The department has a large and distinguished faculty noted for their work in this area, and the graduate program in number theory attracts students from throughout the world. At present over twenty students are writing dissertations in number theory. Each semester upper level graduate courses are offered in a variety of topics in analytic, algebraic, combinatorial, and elementary number theory."  from Number Theory, Department of Mathematics, University of Illinois at Urbana-Champaign

Books on number theory can be found in the Mathemathics Library Stacks on the main level shelved under call number range 512.7 to 512.788.