If I were a Springer-Verlag Graduate Text in Mathematics, I would be Saunders Mac Lane's Categories for the Working Mathematician.
I provide an array of general ideas useful in a wide variety of fields. Starting from foundations, I illuminate the concepts of category, functor, natural transformation, and duality. I then turn to adjoint functors, which provide a description of universal constructions, an analysis of the representation of functors by sets of morphisms, and a means of manipulating direct and inverse limits.
Which Springer GTM would you be? The Springer GTM Test