The Girl Who Cried Monster (Classic Goosebumps #39) by R. L. Stine

Lucy likes to tell monster stories. She's told so many that her friends and family are sick of it. Then one day, Lucy discovers a real, live monster: the librarian in charge of the summer reading program. Too bad Lucy's told so many monster tall tales. Too bad no one believes a word she says. Too bad the monster knows who she is... ... and is coming for her next.