Anything by John Milnor. His little book Morse Theory is a very clear, concise introduction to certain essential aspects of differential topology and Riemannian geometry, starting at a fairly elementary level and winding up with Bott periodicity for unitary groups. In this vein, his Characteristic Classes is similarly clear and concise. To my mind, Milnor is an extremely gifted expositor. From a more scholarly point of view, Kobiyashi and Nomizu's two volumes on differential geometry (can't recall the exact title right now) are pretty comprehensive, both in material covered and in references. And since theoretical physics is within the purview of MO, I think The Feynman Lectures on Physics, vols. I,II,III are a work of real genius. When I was an undergrad at Caltech from 1968-72, we used them for introductory physics; students jokingly called them "the big red sleeping pills" because the material went down so easily it might make one doze off. His Quantum Electrodynamics provides a beutifully intuitive introduction to a fairly abstruse subject. Finally, another of may favorites is Abraham and Marsden's Foundations of Mechanics, both in terms of exposition and scholarship.