？
Were you talking about my book?
Literal
I [of-の] book [of-の] matter [object-を] said [question-の]?
~のこと ('about X, regarding X') is how Japanese frames 'about' for 'talk about, think about, remember about' — literally 'the matter of X.' The sentence-final の functions as a casual question marker, especially in feminine or gentle speech. 言った is past, so 'did you say.'