书肺是一种常见于蛛形纲生物腹部内的呼吸器官。虽然有“肺”之称，但其实与脊椎动物的肺并无联系，只是因为形状相似而得名。“书”一名则来自书肺中的气袋和血淋巴形成的组织。 大多数蜘蛛只有一对书肺，而蝎子一般有四对。也有没有书肺的蛛形纲生物。

书肺的历史可追溯到4.1亿年前，见于已灭绝保存在莱尼燧石中的trigonotarbid生物中。这些泥盆纪化石的书肺很难于现代生物的肺区分。 而关于蛛形纲生物的最大争议之一就是它们的共同祖先是否发展出了书肺。

A book lung is a type of respiration organ used for atmospheric gas exchange that is found in many arachnids, such as scorpions and spiders. Each of these organs is found inside an open ventral abdominal, air-filled cavity (atrium) and connects with the surroundings through a small opening for the purpose of respiration. The oldest book lung found to date belongs to a member of the extinct Trigonotarbida, who lived 410 million years ago and had book lungs already fully adapted to a terrestrial existence.