This trick requires that you own a house with an alchemy lab. Activate the bookshelf, then place the desired book to be duplicated on it. Leave the menu, then immediately activate the bookshelf again. If your book has not yet physically appeared on the shelf, wait at the menu screen where you can loot the bookshelf. Once you see the book in front of you (it will hover in front of the shelf while the menu is open), loot the book from the shelf in the menu. Immediately close the menu, and you can click on the book itself to read and then take it. You will now have looted the book twice, putting two of them in your inventory. Repeat this as many times as desired. It is especially useful with expensive books such as the Oghma Infinium.