Solid wood . Pine, poplar, oak, maple and luaun are the most common solid-wood species at home centers and lumberyards. Some are sold in precut lengths for shelving, but you may have to pick out your own boards and cut them to size. Choose only straight, smooth, square-edged stock. Plane, sand or rout the edges to round over sharp corners, then apply a finish to all sides of the shelf.
Solid wood is beautiful, but it's also prone to warping, twisting, cracking and cupping.
For hardback books and other heavier loads, reduce the span to 24 in. Cover the edge of a plywood shelf with iron-on wood edge banding and you get the look and strength of solid wood with greater stability. You can also use softwood plywood. It's about half the price and just as strong as hardwood plywood, but it does not have the attractive face veneers found on the hardwood version. Particleboard makes good, low-cost utility shelving, but it should be painted for appearance and moisture resistance.