Gateway by Fred Pohl is one of the best known SF novels in existence (although at 40 years, its age may slowly be overcoming its reputation). This novel is the direct sequel to Gateway - a book I read more than 30 years ago.
Robin Broadhead, the protagonist of the previous novel, is older, still filthy rich, happily married, and nevertheless still feeling very guilty about his actions in the previous book. As a result of that guilt, he's obsessed with Heechee technology (the alien race that built Gateway) and funds a trip to a Heechee artifact called "The Food Factory." Part of the book is also seen through the eyes of the Herter family, the people travelling to the Food Factory. A horrible years-long trip, followed by all kinds of discoveries both good and bad.
The writing is acceptable, but not great. The characters are utilitarian. The plot is scattered, split as it is between multiple viewpoints. The mysteries - of what things are, and why they are the way they are - are at least somewhat interesting. But the book isn't nearly as good as its predecessor ... or at least not nearly as good as I remember its predecessor to be, although I suspect I would be more critical of the first book now too. Read Gateway - I'm not sure you need to read this.