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Avoid shelving books too tightly or leaning against one another as this can cause lasting warping, structural damage, and cracking. When shelving, place books of similar height and length next to one another to avoid stressing the spine.  Placing a book flat is an alternative to upright shelving. However, when shelving a book horizontally be sure to put as little weight on top of it as possible.  Grab the middle of the spine when removing a book from a shelf to avoid damaging the head (top) and foot (bottom) of the spine. Place your books in a controlled climate to help prolong the book's quality. Sunlight, heat, and humidity can all have negative effects on the book's binding, structure, and page quality.  Humidity can cause mildew to grow on your books. If you live in a particularly humid environment, and mildew begins to grow, simply wipe the mildew with a very dry cloth and leave the book in sunlight for 30-45 minutes. Be careful not to leave it longer as sunlight can cause the cover to fade.  If a rare or fragile book grows mildew bring it to a specialist in book conservation. Clean your books regularly in order to prevent natural wear and tear and increase the quality of their storage. Dusting your books will help reduce permanent page and cover aging.  Make sure to dust your books from the spine outward so dust does not settle behind the spine.  Avoid storing books in plastic bags. Books need to breathe and plastic can cause molding or warping. Instead, try wrapping the book in acid-free cloth or purchases a book storage box.

Summary:
Shelve books upright. Keep books out of direct sunlight and in a cool area. Keep your books clean.