These were actually two youth novels I read as a kid, but I only need to find one to find the other, unfortunately I only remember enough detail from both books. They were a series involving a young boy who discovers that there is a world or dimension where all the things you lost wind up. For example, if you just misplaced your keys, it was because they actually wound up in this world, and they would be sent back, but usually ended up in a different place in our world. The world there is filled with lost junk.

I believe the title of the first book was just the name of this place “Other World” or something like it. I remember that at the end of the first book, he needs to send the world a milk carton. When he hears his mother say out loud that she can’t find her keys and she had just left them on the counter, he throws the milk carton at the spot where the keys were and the carton disappears into the other world. In fact, I think the cover of the novel had a floating milk carton with milk spilling out of it as if it were in space or something.

If it helps, in the sequel, he winds up back there, but with his annoying little brother, who likes carrying things in his pocket. There was something where a giant clock of some sort runs the entire dimension and without a stuffed bird that sits in a cuckoo nest in the clock, the clock won’t run and the world is doomed. In the end, he remembers that his little brother said the stuffed bird felt the same in his pocket as his favorite golf ball/putt putt ball, so the protagonist saves the day by putting a golf ball in the cuckoo’s nest.