Here are twenty-one suggested ways of arranging the books on your bookshelves.

1. By author’s last name, 2. By title, 3. By subject, 4. By size, 5. Horizontally, 6. By colour, 7. By hardbacks and paperbacks, 8. By publisher, 9. By read/unread books, 10. By strict order of acquisition, 11. By order of publication, 12. By number of pages, 13. According to the Dewey Decimal System, 14. According to the Library of Congress System, 15. By ISBN, 16. By price, 17. According to new and used, 18. By enjoyment, 19. By sentimental value, 20. By provenance, 21. By still more esoteric arrangements.

Taken from The Book On The Bookshelf by Henry Petroski, where you will find a splendid little essay on each arrangement. Petroski also wrote a marvellous book about pencils, called The Pencil.