How do you choose a book, aside from reviews and recommendations? I prefer first lines, first paragraphs. We know many iconic first lines, like "Call me Ishmael." (Not a compelling opening; seriously, try it with another name: "Call me Susan.") 
A first line that delivers anticipation is what I want. When I picked up Eugene F. Walters's novel The Untidy Pilgrim, I knew I was in for a wild ride: "Down in Mobile, they're all crazy, because the Gulf Coast is the kingdom of monkeys, the land of clowns, ghosts and musicians, and Mobile is sweet lunacy's county seat." I give high marks to sentences that promise wit, like "Even a hundred years past the town's founding a visitor to Amicus might guess it had been laid out by rival drunks." (The Exact Nature of Our Wrongs, Janet Peery) 
Some openings are tragic, like Toni Morrison's from Paradise--"They shoot the white girl first."--or "We started dying before the snow, and like the snow, we continued to fall." (Tracks, Louise Erdrich) Some are simply shivery: "Nothing good happens after midnight." (Are You Sleeping, Kathleen Barber). Some start us on a journey: "The train to Odessa is careening along at ninety miles an hour in the green light of dusk, hurdling copper-colored rivers...." (The Fault Line, Paolo Rumiz)--the colors and the verbs promise poetry and adventure, and Rumiz delivers. Some are unexpected, like John Stubbs's in his exemplary John Donne: The Reformed Soul: "His mistress lived with her parents, and access was a problem."
So many great first lines; I have rarely been disappointed in the rest of the book. But every now and then, I choose a page at random and am pleasantly surprised there, too: "How can anything in the world seem bleak when one is eating bacon?" Thank you, Julia Quinn (The Girl with the Make-Believe Husband). --Marilyn Dahl
