Maybe this great book Comparison Theorems in Riemannian Geometry by Jeff Cheeger and David Ebin. For seeing greatness read the reviews of this book:
- ... this is a wonderful book, full of fundamental techniques and ideas. (Robert L. Bryant)
- Cheeger and Ebin's book is a truly important classic monograph in Riemannian geometry, with great continuing relevance. (Rafe Mazzeo)