I've just noticed that there's a whole heap of older Dean Koontz novels going cheap at Amazon UK. A lot of them are probably more properly supernatural/horror or supernatural/sci-fi crossbreeds, but I'm afraid I'm a bit pushed for time to look them all up.
The likes of
The Bad Place,
Midnight and
Shadowfires certainly have a sci-fi element - probably not enough spaceships for some people, though
99p
Watchers
Winter Moon
Cold Fire
Dragon Tears
Hideaway
The Eyes of Darkness
Strangers
Phantoms
False Memory
Icebound
The Voice of the Night
The Mask
By the Light of the Moon
£1.99
Midnight
The Bad Place
Darkfall
Dark Rivers of the Heart
Intensity
Whispers
Fear Nothing
Shadowfires
The House of Thunder
Prodigal Son (Frankenstein: Book 1)
City of Night (Frankenstein: Book 2)
Ticktock
One Door Away from Heaven
The Key to Midnight
The Vision
And one omnibus edition at
£2.99, as well as various other novels (translation, I've run out of time...)
The Dean Koontz Collection (By the Light of the Moon / One Door Away from Heaven / Seize the Light)