It is a rare occurrence for me to read a book straight through from sunset to sunrise, but "The Hollow Man" deserved that kind of attention, even though it is very grim reading--in Dan Simmon's world, it is better to be brain-damaged than normal, drunk or drugged rather than sober, possibly dead rather than alive.
After his wife dies, Simmon's telepathic hero descends into a peculiarly American hell of gangsters, cardboard cities, rabid gangs, child molesters, and serial killers--there are many references to Dante in this book, and although Sartre isn't directly quoted, I'd guess that the main message of "The Hollow Man" is 'Hell is other people.'
What can a brilliant mathematician do when his perfect relationship with his telepathic wife ends with her death? He can commit suicide quickly or commit suicide slowly. Jeremy Bremen tries both ways.
I didn't even try to follow the equations in this book. I had my fill of diffie q's when I was in college. You might be interested in figuring out whether the author is merely waving his hands over the math and science, or whether he is truly attempting to make a case for the creation of new universes every time we commit to an action--whether it is something as simple as sitting down or remaining on our feet. Somewhere in the swiftly branching universes, Jeremy Bremen's wife didn't get brain cancer and die. Another, much older scientist who may be the only one who understands Jeremy's equations, gets the notion that he can find a universe that didn't experience the Holocaust of WWII, and where his family didn't die in the Ravensbruck concentration camp.
He kills himself.
"The Hollow Man" splits its chapters between Jeremy's hellish adventures after his wife dies, and flashbacks to happier days when she was alive and he was solving the mysteries of the universe. The poignancy is almost too difficult to bear, but Simmons is a good author and he makes you want to follow Jeremy to his quietus. Another character, a boy who was born blind, deaf, and mute and who is viciously abused by his mother and her live-in boyfriends, also narrates parts of the story. How Jeremy's universe and the universe of the handicapped boy overlap is the highlight and climax of "The Hollow Man."