I would like to imagine that around this time the dictionary in question was swiped from the library by Cozmo Kramer and it ended up in Jerry Seinfeld’s apartment in the summer of 1989 after the events of Ghostbusters II. Jerry borrowed it to use as the scrabble reference when his parents came to visit NYC.
Uncle Leo and George were busted shoplifting books, but taking from the library seems more like Kramer, and getting the book into Jerry’s apartment from Kramer’s apartment is more plausible.