James M. Cain, Mildred Pierce
Raymond Chandler, The Lady in the Lake and The Long Goodbye
Umberto Eco, The Name of the Rose
James Ellroy, The Cold Six Thousand
Alan Furst, Kingdom of Shadows and Dark Voyage
Joe Gores, Cases
P.D. James, The Lighthouse
Philip Kerr, Berlin Noir
Imre Kertesz, Detective Story
Elmore Leonard, When the Ladies Come Out to Dance (short stories) and Tishomingo Blues (short stories). (Not sure which of his novels to start with.)
Michael Malone, Uncivil Seasons, Time's Witness, and First Lady
Hilary Mantel, Wolf Hall (haven't read the second novel yet)
George Simenon, the Maigret mysteries; try A Maigret Trio for a start.
Robert Louis Stevenson, Treasure Island
J.R.R. Tolkein, The Hobbit and The Lord of the Rings
James Lee Burke, certain of the Dave Robichaux novels; try The Tin Roof Blowdown for a start.
Michael Connelly, certain of the Harry Bosch novels; here are the first three.
Kinky Friedman, Elvis, Jesus, and Coca-Cola
James Hynes, Publish and Perish: Three Tales of Tenure and Terror
Jonathan Lethem, Gun, With Occasional Music
John Mortimer, The Best of Rumpole
Joyce Carol Oates, Mysteries of Winterthurn
Robert Parker, certain of the Spenser novels
Louise Penny, certain of the Inspector Gamache novels
Derek Raymond, How the Dead Live
Scott Turow, certain legal thrillers
Jacqueline Winspear, Maisie Dobbs
Next time you're on a plane, any of these books will give you a couple of hours' escape. Many are so good that you might find yourself rereading them at home. A few would qualify as literature. Note, some are pretty hard-boiled and not all are appropriate for the faint of heart.
One other general note: A good way to identify candidate novels is to look at prestigious awards such as the Edgar. Previous winners of the Edgar award are listed here on Wikipedia.
There's a lot more out there, and I'll update this list from time to time. Feel free to add other greats in the comments. If you do, be very selective—home runs only! Your travel-weary friends will appreciate it.