Without Warning, 1981, by Fern Michaels (a prolific romance novelist whose Wikipedia page doesn't even list this one, lol)
When I first started looking for old horror paperbacks, this wasn't the first one I bought, but it was the first one that awakened me to the joys hiding in the racks. This one was in the lobby of a second-floor bookstore, who had lined the entryway and stairs up with shelves of cheap paperbacks; the delight I felt when I reached in to look at this one and saw SANTA HOLDING A BOMB is indescribable.
The cover is literal, too: this is a book about a psychic girl hunting a killer mall Santa. There's an entire middle passage where she goes and lives in his house for a few hours, which turns out to be irrelevant, bc it's a psychic impression left in another person's photo that cracks the case. There's also a memorable vignette about a sick little kid across the street begging to go to the mall to meet Santa seeing him leaving the bombs on the roof, and waving to him cheerfully
SPOILER ALERT: in a shocking twist, they fail to stop the Santa, and the mall is bombed. Lots of children are dead but a cop's wife had her baby so it all balances out, probably