Mr. Silverfish was a silverfish that first appeared asking Beetlejuice to read him a book called A Doggie called Spot. Before getting to the ending, Beetlejuice has to answer the phone and asks Mr. Silverfish to hold the book for him.
When Beetlejuice returns, the book is gone and Mr. Silverfish burps and laughes nervously, causing Beetlejuice to humorously scream "You ate Mr. Book?" Beetlejuice then recieves a phone call from the library and informs Mr. Silverfish that he owes them a lot of fines (presumably from other books he ate). He proceeds to then tell Silverfish that because of his library fines, he will be "going to Mr. Prison".