The Mystery of the Grinning Gargoyle:
(The Boxcar Children Mysteries #137)

The Aldens visit Goldwin University, where Grandfather attended college. The old library is about to be torn down. But legend says the stone gargoyles guarding the roof of the old building will do anything to protect it--even fly. When the creatures begin appearing outside the windows, people are terrified. But the Boxcar Children think there is another reason the gargoyles are turning up in places they shouldn't. And they're determined to figure it out.