_This Is It_ by Alan Watts is a good solid work, but is too short to provide the rigorous instruction needed to achieve any appreciable amount of enlightenment or Zen Satori. That said, it is a wonderful, enjoyable, and profound book page for page, and is essential to round out your collection of Alan Watts' more recent and well-known works. _This Is It_ is also perfect for someone who does not want a megadose of strenuous philosophy and theology; it is ideal for those who are new to psychological-religious non-fiction, or who do not have the time to hack through some kind of magnum-opus epic of philosophy. If, however, you want to read one of the more comprehensive books by Alan Watts, I would recommend _Psychotherapy East and West_, which is his best work.
Alan Watts explains what unity/non-dual consciousness or nothingness really is and explains how it may be different from what many people think it is. He uses various interesting examples to make his case and I thought they were very effective. Ken Wilber has elaborated on this error that people tend to make as well but I think Toru Sato's "Rhythm, Relationships, and Transcendence" really explains it the best. Anyways all of these author's books are well worth reading. They are all brilliant! We are very fortunate that these people write books.