Narrative hook

A narrative hook (or just hook) is a literary technique in the opening of a story that "hooks" the reader's attention so that they will keep on reading. The "opening" may consist of several paragraphs for a short story, or several pages for a novel, but ideally it is the opening sentence in the book.
Examples.
Opening a novel with startling, dramatic action or an ominous description can function as a narrative hook. Ovid's "Fasti" employs narrative hooks in the openings of each book, including a description of a bloody ghost and an ominous exchange between the characters Callisto and Diana. 
A narrative hook can also take the form of a short, often shocking passage discussing an important event in the life of one of the work's characters. The device establishes character voice and introduces a theme of the work. In Anna Quindlen's "Black and Blue", the opening sentence recounts the first time the protagonist endured abuse from her husband, which is the core theme of the novel. Opening lines that introduce an important event without providing specifics, such as "And then, after six years, she saw him again." from Katherine Mansfield's "A Dill Pickle", pique the reader's curiosity and encourage the reader to discover the answers later in the work.
The "in medias res" technique, where the relating of a story begins at the midpoint, rather than at the beginning, can also be used as a narrative hook. Toni Morrison's "Beloved" begins "in medias res" and transitions to a description of the house that serves as the novel's setting, disrupting the reader's expectations of a typical narrative structure.
A thematic statement, as with the opening line of Jane Austen's "Pride and Prejudice" ("It is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife."), can also serve to hook the reader's attention.