I can't find any artist reference on any of the multiple copies of McIntyre's Wrath of Khan
novelization in my collection (Pocket 1st, later Pocket, Gregg Press, Macdonald, SFBC...) but I'm pretty sure the cover art is by Boris. The style seems pretty unmistakable. I sent Vallejo a message on his website, asking if he painted it. I'll report back when (if?) he responds.