The crying of lot 49 / Thomas Pynchon.

This is considered a postmodernist novel. The protagonist is a woman named Oedipa Maas who, when the novel begins, learns that her former boyfriend, the wealthy Pierce Inverarity, has died and designated her to be the executor of his enormous estate. Inverarity's assets include vast stretches of property, a significant stamp collection, and many shares in an aerospace corporation called Yoyodyne. As Oedipa goes through her late boyfriend's will, aided by a lawyer named Metzger who works for Inverarity's law firm, she learns about a series of secret societies and strange groups of people involved in a sort of renegade postal system called Tristero. [She leaves her husband and heads for Southern California.] She starts seeing ubiquitous cryptic diagrams of a simple horn, a symbol with a seemingly infinite number of meanings. Every clue she uncovers about Tristero and the horn leads haphazardly to another, like a brainstorm, or a free association of ideas.