For shelves and bookshelf constructions, use 3/4-inch thick materials. If you're using hardwood solids, you may increase the thickness to 1 1/4 inches for nearly limitless support. If you want to go even thicker, consider using 2 inches wood. The ideal bookcase width is between 39 and 42 inches, but you can get by with a little less or more space depending on your needs. Generally, fill any voids between shelves with filler items such as decorative plates or books. Shelf depth is determined by what type of material you choose. If you go with solid wood, go for at least 1 1/4 inches to ensure adequate support. For plywood, consider using 3/4 inch thick materials if you plan to only fill in the middle with books or clutter. As mentioned, the ideal height of a bookshelf is between 36 and 42 inches, but that's just because that's how much room most people have under their desks. You can make lower or higher shelves provided you have enough space above them. In fact, some bookshelves are built with additional storage space underneath for yet more shelving. It's best to buy wooden shelves from a manufacturer who specializes in wood products. This will help ensure they're well made and won't break down quickly.