The book is Bats at the Library, written and illustrated by Brian Lies.
Bats at the Library is about a group (flock? -- no, it's a colony) of bats who find an open library window and spend the night exploring everything the library has to offer. They read, they talk about their favorite books, they play games, and they even have storytime. I really did love this book. The text is rhythmic and clever, and the illustrations are filled with humor and detail.
Of course, you don't have to take my word for it (a small homage to the now canceled Reading Rainbow). Bats at the Library has received excellent reviews.
From Booklist, "The rhymed narrative serves primarily as the vehicle for the appealing acrylic illustrations that teem with bats so charming they will even win over chiroptophobes. -- Perkins, Linda. Booklist; 10/15/2008, Vol. 105 Issue 4, p46, 1
From School Library Journal, "Lies's acrylics are a successful fusion of fantasy and reality. The rhyming narrative is generally smooth, with enough humor and sophistication to propel readers along. And who can argue with the message? --Lukehart, Wendy. School Library Journal; Nov2008, Vol. 54 Issue 11, p92-92, 1/5p
Bats at the Library could be used in a storytime, but I really see it as an introduction to the library for young children. Children are like the young bats in the story who see the library as a playground, but the older bats know better. Young or old, they all get pulled into a good story.