。
I'm reading a book.
Literal
Book [object-を] read-[ongoing-ている]-[explanation-んです].
A polite explanation — the speaker is explaining what they're doing, likely in response to a question like 'what are you up to?'. ~んです adds a tone of providing context or explanation, marking the statement as background rather than just neutral information. Without it, the sentence would be 読んでいます — equally grammatical but less natural as an explanatory answer.