Elm Planks For Good Shelves

Too often I've bought sheets of plywood in order to construct shelves of one sort or another. This time, though, to make some free-standing shelves with decent span I went to our local wood yard (Richardson's in Mill Lane, North Walsham) and was sold a lovely pile of elm. They are all 2.3m long and the narrowest piece is 38cm wide. Each plank weighs around 20kg, so if I use the lot it will be 120kg of shelving.