Coben has published more than thirty bestselling thrillers, and he's plotted almost every one without a full outline. He starts each book knowing two things for certain, where it opens and where it lands. The middle he figures out as he types.

The Direct Answer
He doesn't outline. Coben locks his ending before he starts, then writes toward it and discovers the middle along the way.
He'll sometimes sketch a chapter or two ahead, but that's as planned as it gets. Per his 2015 HuffPost interview.

Two Fixed Points, No Map Between Them

Coben begins a novel knowing where it opens and where it ends. Everything between those points he invents at the keyboard. In a 2015 interview about The Stranger, he said he knows the beginning and the end but not much of the middle, and that he'll occasionally outline a chapter or two ahead when he needs to. That's the whole system. The uncertainty is deliberate, because if he doesn't know what's coming, his readers won't either.

When I start to write, I know the beginning and the end, but I don't know much in between.

Harlan Coben, HuffPost, 2015

The Drive From New Jersey to California

His favorite way to explain the approach is a road trip. He's said he might take the direct highway, or he might detour through the Suez Canal and stop in Tokyo, but he always ends up in California. The destination is fixed. The route is whatever the story demands. That's what lets his plots swerve so hard, since he's finding the turns at the same moment his readers will.

He often points to a line he attributes to E.L. Doctorow, that writing a novel is like driving at night: you can see only as far as your headlights, but you can make the whole trip that way.

This ending-first habit shapes his entire process. See the full Harlan Coben writing routine for how it fits his book-a-year pace.

What You Can Steal