What do Sudoku puzzles have in common with library books? — They both involve sequences of numbers! While most library call numbers that we use here for our nonfiction books aren’t as long as a nine-digit sequence in Sudoku, for topics with lots of books, we do have some call numbers as long as eight digits. You’re mosty likely to find these long call numbers in travel (call numers Travel 914-919), true crime (364), cooking (641), music history and criticism (781-782), TV & film history & criticism (791), and WWII (940.5), though they’ll crop up other places, too.