Kitap + alt → Kitabın altı (the underneath of the book)
The -ın is not the 2nd person possessive there.
Kitabın (your book) + alt → Kitabının altı (the underneath of your book)
Kahve kitabının altında. = the coffee is under your book.
Locative on normal nouns is just "-da", but if the noun has the possessive suffix (alt-ı), we add an "n" to separate the two cases. That's just a rule.
It is difficult to work out, when the sentence is so strange. Coffee is under the book! What?? Why? Confusing.
You would have to include "bir" in this instance. :) The genitive case kind of implies that it is specific.
The typo detection bugs me. Sometimes I'm totally wrong and it sees a type. But when I spell coffee - coffe, it doesn't :)
These sentences are so nonsensical that they seem to be with the intent to not help you learn anything. Why not use practical sentences that can be applied in more but the most niche and obscure circumstances?