Being and Nothingness is a philosophical work by Jean-Paul Sartre, published in 1943 during the German occupation of France. While perhaps not the best or most germinal work on existentialism, the book is probably the basic systematic work on existentialism available to the general educated public.
The book can basically be divided into two parts. In the first, Sartre explains his basic philosophical concepts. We start with Being, which is basically revealed as an "In-Itself", a facticity that is beyond either affirming or denying. Against this is human consciousness. Human consciousness separates itself from the In-Itself through its ability to envision nothing. Through this "nothing", which is radically different from the In-Itself, consciousness becomes a "For-Itself", a type of Being that is on a different order than that of the In-Itself. In other words, the For-Itself, through the process of existing, separates itself from the essence of the In-Itself. Thus, the philosophy is called existentialism. Those who are unwilling to accept the contingency of their "For-Itself" nature are in Bad Faith.
All of this is described in philosophical terms that are both technically nuanced and illuminating, and it is this part of the work that is probably Sartre's important contribution to 20th century philosophy.
The book is a long book though, almost 800 pages in my translation. The later, and lengthier part of the book, is where Sartre takes his newly shaped philosophy and launches it like a wild pinball into psychology, sociology, sexuality, politics and any and every other subject that he might wish to pontificate on. These sections seemed much less useful to me, seeming to be merely an excuse for him to comment on whatever crossed his mind. Given how radically our views of psychology (Freudian analysis at the time) and sexuality have changed, these sections seem truly dated and somewhat frivolous to me, as opposed to the clear presentation of the basic philosophical themes. The book actually concludes with a long essay discussing why water skiing is more psychosexually fulfilling than snow skiing.
In some ways, the image of Jean-Paul Sartre is built up so much around other facets of his career, such as his famous play No Exit and his regrettable endorsement of Stalinism, that his works as a technical philosopher is sometimes overlooked. Being and Nothingness shows both sides: a quite important philosophical work, mixed in with some very stereotypically French overwrought writing on the meaning of sexuality and sensuality and society, etc.