《Lectures on the H-Cobordism Theorem》

Milnor's "Lectures on the h-cobordism theorem" consists of a proof and applications of the h-cobordism theorem, an important technical result that, among other things, leads immediately to a proof of the Poincare conjecture for smooth manifolds of dimension >= 5. The theorem was originally proved by Smale in 1962 (part of the basis for his Fields Medal) using handlebody techniques, but in this book Milnor presents a (partially) different proof using Morse theoretic lemmas due to Morse and Barden. Along the way, a number of theorems and techniques in differential topology are used or derived (including the Whitney trick, finger moves, bicollarings, surgery, extensions of embeddings and isotopies, smooth structures on unions, Poincare duality), which makes this valuable as much for the methods as for the final result. Definitely it is one of the best books for learning how to actually prove things in topology, mixing in Morse theory, Riemannian geometry, and algebraic topology, too.