Martin Heidegger's Being and Time (1927) is one of the most rigorous and methodically constructed treatises you will find anywhere in philosophy. Heidegger is known for his difficulty, but this book holds an added challenge due to its cumulative dependence. What I mean is, you cannot possibly come to fully grasp the later sections of the book without grasping earlier sections. Every bit of the author's impressive terminology (whether it be a common term imbued with new meaning or a clever neologism) is systematically chosen, introduced, questioned, and developed as the text moves along. If you have never read Heidegger before, expect to reread certain paragraphs at least twice before their meaning begins to dawn on you. If you are persistent, the meaning of his precisely formulated sentences will cause you to perceive the world in entirely new ways. I recommend - at least at first - taking this book in small doses. If you feel your mind wandering at all just stop and go back to it later. If you are looking to scan this book for tidbits of wisdom you will likely be disappointed. Alternately, I would plan on a commitment of at least a few months if you want to glean anything at all from the text.
For those who have read Heidegger before: this book is definitely his magnum opus. Within it he establishes a point of departure for all of his later thought and works. It is also the most engaging and enlightening read you will encounter in his repertoire. Compared to Heidegger's post-kehre writings, you will find the material and style in Being and Time to be far more precise and clear [a very difficult feat indeed considering the elusive nature of the subject matter]. Also, reading this book more than once is a must! Do not be surprised if after the first read you feel as though you are missing something - you probably are.
For those who have read Heidegger before: this book is definitely his magnum opus. Within it he establishes a point of departure for all of his later thought and works. It is also the most engaging and enlightening read you will encounter in his repertoire. Compared to Heidegger's post-kehre writings, you will find the material and style in Being and Time to be far more precise and clear [a very difficult feat indeed considering the elusive nature of the subject matter]. Also, reading this book more than once is a must! Do not be surprised if after the first read you feel as though you are missing something - you probably are.