What is Linen?
Linen is one of the first fibers that people made into string and cloth. Linen comes from the flax plant, which grows all over the Mediterranean region and Central Asia. Flax is a tall, reed-like plant, with long fibers which make it easy to spin into thread. You pick the plants, and then leave them to soak in a tub of water or a stream until the hard outside stem rots away and leaves the long, soft fibers underneath. People call this retting the flax.
Then you take the fibers and spin them on a spindle into linen thread. You can spin linen into a thick, strong thread, or you can spin it very very fine, depending on the skill of the spinner and what you want to use it for. People in Egypt made sails out of coarse linen, for example, but used very fine linen for expensive tunics. It is hard to dye linen, so mostly people wore it white, the way it is naturally. Linen is not as warm as wool, but it is much softer and more comfortable on the skin (after you wear it a while; at first it is stiff and scratchy).
In Central Asia, people were spinning linen thread and rope by about 30,000 BC, long before wool. It took longer to invent weaving, but by 5000 BC, at the latest, people were weaving linen into fabric. In the first millennium BC, the Egyptians mostly wore linen, while Greeks and West Asians and Germans mostly wore wool. By the Roman period, however, many people wore linen tunics for comfort with wool robes over them for warmth, and in the Middle Ages in Europe this continued to be common, so that "linen" got to mean something like "underwear". Our word "lingerie" is related to linen. In the Islamic Empire, on the other hand, people began to wear mainly linen and cotton, and not so much wool.
Eyewitness: Costume, by L. Rowland-Warne (2000). For kids, but mainly European clothing, from earliest times to modern.
World Textiles: A Concise History, by Mary Schoeser (2003). For adults.
Women's Work: The First 20,000 Years : Women, Cloth, and Society in Early Times, by Elizabeth Wayland Barber (1995). Not for kids, but an interested high schooler could read it. Fascinating ideas about the way people made cloth in ancient times, and why it was that way.