The Doors of Perception and Heaven and Hell is actually two essays really put together as a book. The first The Doors of Perception is essentially a mescaline trip report with an attached perspective on how this reflects on the everyday world. The second Heaven and Hell is concerned with questions of death, afterlife, Tibetian Book of the Dead stuff.
The main point of the first part is:
To make biological survival possible, Mind at Large has to be funneled through the reducing valve of the brain and nervous system.
The concept of brain as "reducing valve" has been continued in a lot of theory of mind work based on the usage of psychedelics. It is an interesting worldview, basically following in the lineage of Plato. It is hard to have a psychedelic experience and not feel that something about "normal" life is reduced from what is really going on.
The other theme was around how these experiences can be related madness:
The event was this succession of azure furnace doors separated by gulfs of unfathomable gentian. It was inexpressibly wonderful, wonderful to the point, almost, of being terrifying. And suddenly I had an inkling of what it must feel like to be mad.
Overall this book is a good, short read that is often cited in later works on the psychedelic experience. Therefore if for no other reason than you might hear references to it this is a worthwhile read.