。
The books in this room aren't mine.
Literal
This room [genitive-の] books [topic-は] my [genitive-の] thing is-not.
A beginner-level sentence showing the possessive construction. 私の物 ('my thing') is the standard way to say 'mine' in Japanese — Japanese lacks a dedicated possessive pronoun and instead uses [possessor]の物 ('X's thing') or simply [possessor]の. The ではありません is the polite negative of だ, the full polite-register opposite of です. The sentence illustrates the topic-comment split cleanly: set up 'the books in this room' as the topic, then deliver the verdict 'are not mine.'