These books were channelled by a woman named Jane Roberts. Seth was the name of the entity who spoke through her. He had interesting views on spirituality as well as the construction of the universe. Particularly regarding unquantifiable energy. He taught that this world being made up of pure energy exists as we percieve it because we as spiritual/energetic beings have agreed on the basic makeup of it. Also that we do reincarnate but that time is not linear and that all lifetimes that we experience are simultaneous. And that we can sometimes slip into and out of these other lives during sleep. There are quite a number of Seth books. Jane Roberts channelled him for quite a number of years.
The books are quite fascinating and I am always interested in new perspectives. Some of the reading seems to explain certain mysteries and some of it creates more. Worth the reading though.