Sufism refers to the more spiritual side of Islam, Islamic mysticism, and the people who practice it. Most experts on Sufism start their books by trying to explain where the word came from. They usually mention a few possibilities.
1) The People of the Bench (Ahl as-Suffa, اهل الصفة)
2) The word for wool (Sūf, صوف).
3) The word (Safā, صفا), meaning, "to be pure."
The Ahl as-Suffa were a group of Companions of the Prophet Muhammad who gathered at a section of the mosque in Medina. They were poor, at the time, and involved in spiritual practices.
Sufis became known by the wool garments they wore, made of a more coarse and uncomfortable material.
Sufis are pure, or trying to be pure. Fasting, little sleep, little talk, much dhikr (ذكر) "remembrance" of God. They aim to purify their hearts so they will act as mirrors for the attributes of God, Allah, the Merciful, the Compassionate.
The experts say it probably originated from the word for wool (Sūf, صوف), but no one really knows, and God knows best. But the alternative explanations themselves are now a part of the whole culture of Sufism.
The real knowing that Sufis are after is the felt, present experience of God, something that can't be put into words or described. But something that is undeniable to the Sufi, to the one who experiences it.
A thousand years ago, Muslims and Sufis could be found everywhere between Morocco and China. Turks and Persians became Muslims and also scholars of Arabic. There was intense cultural interaction as peoples became Muslim. There were Sufi Muslims in China who were educated in Chinese, Taoism and Confucianism, and also Arabic and the Sufism of Ibn 'Arabi, from Spain.
Sufism developed in many layers, philosophically, theologically, institutionally, musically. Sufis have been criticized by "purist" Muslims for adopting practices they claim to have come from the non-Muslims...from Buddhists, Hindus, or Christians. Sufis have been praised for the same reason, by Christians happy to claim some influence on Rumi, or by Hindu Indians who claim Sufis learned spiritual practices from them.
Others deny that Sufism came from outside influences. They claim it is the result of sincere prayer and contemplation of the Quran. The works of the early and later Sufis themselves are with us today. Have a look and judge for yourself.
However, Sufism isn't really in books. You'd be better to find a Sufi. Listen to the sound of the ney (reed flute). Or find a group of Sufis and join their dhikr.
As Rumi said at the start of his six volume Sufi work, "No raw will understand the cooked. So let's cut our talk short and say so long."