Satan Claus is Coming to Town by Catherine Lievens

Dear Satan Claus…
That’s how the letter Satan received starts. The little girl who wrote it asking for her father to be happy again meant it for someone else, and while that’s obvious, Satan doesn’t care. Santa will never find out, as long as Satan’s nosy assistant, Jasper, doesn’t tell him. Satan isn’t sure why he decides to watch Zoe and her dad, except that he’s bored and lonely. He surprises himself when he moves into the house next to theirs and introduces himself as Sam—because he couldn’t exactly give Riley his real name, could he?
Riley has been divorced for years, and he doesn’t have time for a relationship. He has trust issues and no will to get over them, not when his daughter is the center of his life. Then a new neighbor moves in, and he’s adorably awkward and sexy. Riley tries to keep Sam at arm’s length, but he can’t help but fall in love with him, and he’s pretty sure Sam is falling for him, too.
Then Sam confesses his real name is Satan, and that he’s the king of Hell.