The Key to Midnight is Dean Koontz's first great novel, back in 1979, written under his brief pseudonym Leigh Nichols. He went back several years ago and reworked it and republished under Koontz.
It's really good stuff. He says it was his first, perhaps only, foray into some international intrigue along with the usual dark, psycho thrills and awesome twists.
The ending got me. Couldn't figure out where he was going until I got there.
Giving it a solid 7 on the Haugenometer. Goodreads gave it a 3.9 of 5. Got me in the mood for some Ed Poe.