The Ruins by Scott Smith.
I’m not familiar with that Jack Sparks.
In “The List of Seven” the two main characters are a fictionalized Arthur Conan Doyle, and Jack Sparks who is a sort of detective/secret agent for the Crown. By the end of the book Sparks becomes Doyle’s inspiration for Sherlock Holmes.
And even though it doesn’t exactly pass all the requirements set out by the OP, I would still recommend it. Mark Frost, the author, was co-creator of “Twin Peaks” with David Lynch, so as you might expect there is plenty of weird creepy stuff going on.
Loved that novel (along with its sequel, The Six Messiahs), but yeah, I didn’t get a real “horror novel” vibe while reading it (even if I could detect some Lovecraft-esque elements),
Sorrow’s Knot by Erin Bow. It’s the last book I stayed up late to read. Very interesting monsters.
Another outstanding one (which I read in one day) is Mapping the Interior, a novella by Stephen Graham Jones. A boy sees his father’s ghost and…well, describing it any further would spoil it, I think.