Octav Cornea 
Lusternik-Schnirelmann Category [PDF ebook] 

Lusternik-Schnirelmann category is a subject with ties to both algebraic topology and dynamical systems. This book provides a unified approach to LS-category, including foundational material on homotopy theoretic aspects of the subject, the Lusternik-Schnirelmann theorem on critical points and more advanced topics such as Hopf invariants, the construction of functions with few critical points, connections with symplectic geometry, the complexity of algorithms and category of $3$-manifolds. This is the first book which takes LS-category as its central theme and develops topics in topology and dynamics around it. As such, it leads from the very basics of the subject to the present-day state of the art. The prerequisites for reading the book are few: two semesters of algebraic topology and, perhaps, differential topology.