Well, in Finnish, nouns are generally declined instead of using postpositions for that kind of stuff:
kirja pöydällä - the/a book on the/a table
kirja pöytä+llä
book table.supe
You could say
kirja pöydän päällä ("the book on top of the table"), but at least informally in most contexts that'd sound kinda weird unless it was relevant to clarify that the book was on top of the table rather than somewhere else; you could say
pöydän päällä oleva kirja ("the book that is on top of the table"), but again, in most contexts that'd sound pretty formal and could easily make you sound like a foreigner. In writing, of course, that's not nearly as much of an issue.
Also, a sentence like "kirja koiran päällä oli Stephen Kingin
11/22/63" ("the book on top of the dog was Stephen King's
11/22/63" or "the book on the dog's head was Stephen King's
11/22/63") would be perfectly normal (at least grammatically) because using
koiralla here could easily be interpreted as the book being held by the dog or even being at the dog's place (if a doghouse is treated as being owned by the dog; it's generally not AFAIK, so the latter wouldn't really make sense).
But with a table, wording it like that in an everyday conversation without sounding at least somewhat stilted would be really context-dependent because tables are inanimate objects and as such can't "have" things, meaning there's no way any native speaker could mix up the identical declensions in that context, so using
pöydällä would be simpler and as such preferred. There could be tons of contexts where you could still say
pöydän päällä, but in all contexts I can think of, they'd be interchangeable.
Actually, I have no idea what the cases are properly called or what the logic behind them is... supposedly they're only superessive and adessive, but that's probably not true. I'm an idiot, though, so hopefully some other Finn can explain that. All the different cases that are identical make sense intuitively with the differences being obvious, but I couldn't explain them or tell which one is used in some contexts if my life depended on it...
Anyway, here are the others as well:
mies Saksasta - the/a man from Germany
keskustelu meistä - the/a conversation about us
kirja hänen elämästään - the/a book about his/her life