Ellen and Corey Series by Peg Kehret

Ellen Streater received the perfect birthday present—an overnight camp-out at the zoo. Too bad she had to invite her little brother Corey along. But as night falls in the zoo, her brother becomes the least of her worries, as they discover they're locked in the zoo with an escaped convict who's stalking their every move. Unable to run, and with no help in sight, their last chance for survival is Ellen's crazy science project on animal communication... moreEllen Streater received the perfect birthday present—an overnight camp-out at the zoo. Too bad she had to invite her little brother Corey along. But as night falls in the zoo, her brother becomes the least of her worries, as they discover they're locked in the zoo with an escaped convict who's stalking their every move. Unable to run, and with no help in sight, their last chance for survival is Ellen's crazy science project on animal communication. But can they get the animals to help them in time, or will it be too late? "A compelling and fast-paced mystery with characters that are believable and well drawn." — The Horn Book"Plausible incidents and believable characters combine in a fast-moving and well-constructed tale." — Booklist less

While acting in a "haunted house" fundraising event, Ellen is contacted by the ghost of the mansion's first mistress.