Probably the best description of what this book is about comes from the subtitle, 'An essay on phenomenological ontology'- its a thorough analysis of the nature of existence from the point of view of human consciousness. Sartre begins with our most basic knowledge and works his way up to the complexities of human relationships, leaving nothing out. The first Part (of four) of the book centres around the two fundamental components of consciousness. Being is what we are aware of as existing; and Nothingness signifies any kind of negation, such as what we identify as missing, or even the giving of boundaries to an object. Consciousness is shown to be the agency responsible for introducing nothingness into the world: it is we who decide where the boundaries lie or who notice a component missing from the whole. Hence Sartre distinguishes two species of being: in-itself, i.e. a fixed, definable object in the normal understanding of the word; and for-itself, something with free will and which, therefore, is constantly moving beyond what it is was towards something new. Part II deals in-depth with the for-itself, Parts III & IV move on to relationships between for-itselves. One of the other reviews condemns Sartre for lack of argument. In fact, there is nothing to argue for, this book is a description, Sartre regards knowledge derived from closer scrutiny of the subject matter as superior to that elicited by chain of reason. Actually, the misunderstanding here is fundamental, and boils down to the conflict between the analytic and continental schools of philosophy.
This book is anti-religious, anti-scientific and anti-analytic. These three facts are the reason for a lot of general abuse that is hurled at the book, Sartre, and continental philosophy as a whole. However, with an open mind you will find that Sartre makes a very strong case for himself. The prose is difficult to follow, there is no disputing that, however, it is premature to dismiss the work as impenetrable, or even 'confused and obtuse'. I am an A-level student with no formal background in philosophy and I found no insurmountable problems. Continental philosophy is almost always like this, and the reason for it lies in the nature of the content. Sartre is attempting to describe something that precedes logic, and if you are willing to accept that such a notion is possible you will realise that it makes redundant the use of clear-cut definitions and logical language structure. Instead, Sartre must resort to using almost poetic descriptive methods (paradox and oxymoron abound), and the result of this is that the reader must take a more active approach to the understanding of the text. You have to 'think around' the words and sentences to find a meaning that is coherent. Sometimes this requires adapting your interpretation of earlier material. Having read to the end, I can assure you that there is a least one way of understanding what Sartre is trying to put across, although it is a time-intensive endeavour. I would recommend reading a short introductory book, some of Sartre's fictional works or Existentialism And Humanism, to get an idea of what you're in for; you're liable otherwise to regard yourself as having wasted a considerable amount of life-time and 13 quid on top of that. Also, Sartre kicks off with an immense amount of jargon, if you are without a basic grounding in philosophy, something like Penguin's Dictionary of Philosophy will prove useful.
It's not an easy read, and I can't agree with everything said, but for the most part it is incredibly perspicacious. It is written with an intensity that simultaneously demands and enthrals. Sartre's philosophy answers a lot of questions very well, and if you are both interested and determined enough to want a full account of his thought, this book is wholeheartedly recommended.