Sometime in the mid- to late-2000s, I remember reading a book that was about an organization of extradimensional beings who control the minutiae of the universe. I remember very little about it, but this is what I do recall:

• It's a young adult novel
• Main character is a human recruited by said extradimensional beings
• They control basically every aspect of physics, like making sure to fill up the rainclouds with the appropriate amount of water and lightning
• Seconds (as in the unit of time) exist as a physical object, there are also firsts and thirds
• There may have been some pun between splitting the atom and the phrase "split second"?
• The world these beings inhabit is the place where all mysteriously lost objects end up, I think. There's a "barrier" between the worlds that's only visible from their side.
• The general tone of the novel is absurdism, but I'm pretty sure it has its serious points.

Does anyone have any idea what book or series this might be?