In Fahrenheit 451, who found Montag's books and what did she do?
This question depends on where you are in the book.
If you are in the first chapter, Mildred finds a single book that Montag is hiding behind his pillow when Beatty is there at the house. A series of struggles occur between Mildred and Montag demonstrating that he is hiding something, but Mildred never at that point reveals the book to Beatty.
In the end of the first chapter, Montag removes the ventilator grille and a pile of books fall on he and Mildred as he encourages her to journey with him in discovering what meaning lies behind these items. She shows minor interest, but Montag is begging for her approval and partnership.
By the beginning of the second chapter, some of the women are at the Montag's home. Montag tries to gather their support for books, but at Faber's advice, Montag plays it off as a joke. In this situation one woman cries because he reads something rather painful in the poem "Dover Beach".
During the beginning of the third chapter, Mildred has called in an alarm. She is leaving the house when he arrives to the call.
Depending on the she (Mildred or her friends) you are referring to, you should now have a variety of actions to choose from to answer this question.