You ask about Mrs. Auld's response to seeing Douglass with a book. However, the interaction in question didn't actually involve a book. Mrs. Auld became very angry when she caught Douglass with a newspaper.
In any case, this reaction represented a dramatic shift in Sophia Auld's treatment of Douglass. When he had first come to live with her, she had been kind and warm. She had taught him the alphabet, and helped him learn to spell short words. But when her husband discovered the lessons, he ordered her to stop. He explained to her that was both illegal and "unsafe" to teach a slave how to read. It would "spoil" him:
"He would at once become unmanageable, and of no value to his master. As to himself, it could do him no good, but a great deal of harm. It would make him discontented and unhappy."
Afterwards, Sophia Auld changed her attitude. As noted in chapter VII, she ceased instructing Douglass, and ended up becoming "even more violent in her opposition than her husband himself." Douglass cites this example:
"Nothing seemed to make her more angry than to see me with a newspaper. She seemed to think that here lay the danger. I have had her rush at me with a face made all up of fury, and snatch from me a newspaper, in a manner that fully revealed her apprehension."
Douglass also notes that she tried to avoid leaving him alone (in case he'd seize on the opportunity to look at a book):
"From this time I was most narrowly watched. If I was in a separate room any considerable length of time, I was sure to be suspected of having a book, and was at once called to give an account of myself."
Ultimately, her efforts fail. Douglass manages to continue his education, teaching himself to read and write on the sly. In fact, Douglass notes that it was Mr. Auld's warning that inspired him "with a desire and determination to learn." By showing their opposition, the Aulds helped convince Douglass that literacy would indeed transform him, and help Douglass escape bondage.