Your child won't go to sleep, and you've stopped being able to say anything about it that helps. These are the six books for that. Any single one of them does the job. Pick by your child's age and by which fear it actually is, because the dark and the closet are two different problems. And here's the honest part, since somebody should say it out loud: most of these books put a monster on the page. That's how they work, and it's also the risk. Every one below tells you plainly whether the monster gets defused or just gets survived, and which of these you can safely read with the lamp already low.
Take one off the shelf, whichever one matches. The first question is which fear it is. If it's the dark itself, the plain unlit dark, then Can't You Sleep, Little Bear? is a good place to start, and it's the only one with nothing at all in it, so it's also the one you can read at lights-out on a bad night. The Dark and Orion and the Dark take the same fear for an older child and turn the dark into somebody: Snicket's version is quieter and stranger and asks a bit more nerve, Yarlett's is loud and funny and friendly from the first page. If it's monsters instead, Go Away, Big Green Monster! is two minutes long and lets a three-year-old do the shouting themselves, and There's a Nightmare in My Closet is the old Mercer Mayer joke where the thing in the closet turns out to be a crybaby who needs tucking in. And if what actually happened is that a story got in at the wrong hour, The Berenstain Bears in the Dark is that exact evening, older sibling and all. Read each book's heads-up before you choose. On a shelf about being frightened at bedtime, the difference between a monster that gets befriended and a monster that just leaves is the whole thing.