YES!! You feel bad and it was an honest mistake, so I'm sure she will take it well! Just make sure to explain everything, and DON'T finish the book!!
Also, if you didn't tell her, think about how much worse it will be if she finds it/ out without you telling her. Kinda suspicious, right??
But don't sweat it! You've got this, and your mom will 100% appreciate your honesty! :D