Harper Perennial Modern Thought What Is Called Thinking?, (Paperback)

This book, a series of lectures, is obscure even for Heidegger. The best preparation, other than knowing a whole lot of classical philosophy and modern philosophy (especially Nietzsche), is to be familiar with eastern thought. Heidegger develops his own terminology as he interacts with western philosophy, but his explanation of thinking owes more to eastern concepts of duality than anything else. (Might want to start with the last two lectures where he gets to the point.)