When you're buying bookshelves, appearance and cost are usually the first considerations, but it's equally important to think about the weight, number, and size of the books you intend to store. If you have a small collection of paperbacks, for instance, a two-shelf knockdown case is all you need. But if you have a substantial collection of oversize home-decorating, gardening, or art books, you'll need a unit with shelves that are deep (12 inches or more) and high (15 inches or more).

The first decision you have to make is where the bookcases will go. Regardless of whether they're ready-made or custom crafted, bookshelves need to be positioned where the load can be properly supported. Bob Maloy, whose San Francisco-based business, System Concepts, Inc., builds libraries for law firms and institutions, says it's safest to place bookshelves along a perimeter, or bearing, wall, which can share the load with the floor. Make sure freestanding shelves higher than four feet are equipped with anti-topple devices, such as adjustable feet or shims (thin pieces of wood placed under the feet to level the unit), and that they are anchored to the wall at the top.

"Drywall is not structurally sound enough to provide anchorage," says Maloy. Anchor bookshelves to studs, which you can find by tapping a small nail along the wall until the wall no longer sounds hollow. Or use a device called a stud finder, available at hardware stores. Once you've found one stud, you should find the others at intervals of about 16 to 18 inches.

After determining the best place for the bookshelves, figure out how many feet of shelving you need. Stack books on the floor and measure in groups. "I always tell clients to overbuild," says William Feeney, owner of William Feeney Custom Furniture Design in Brooklyn, New York. "I like the look of empty shelf space, and book lovers tend to keep amassing more. And it's nice to have the option of putting a knickknack on the shelf." The shorter and thicker the shelves, the sturdier they will be. Three-quarter-inch shelves should be no longer than 24 inches; one-inch shelves can run to 36 inches.