The term leishu 類書 is usually translated as "encyclopaedia", even if Chinese books of this genre and type do not fully correspond to the Western term, and also show differences among themselves in arrangement and composition. Generally spoken, leishu present an encylopaedic overview of a wide range of themes, often with the claim to cover all topics of the three realms Heaven (incl. astronomy), earth (geography, flora and fauna), and man (human relationship, politics, etc.). There are also leishu specializing on certain topics, like medicine, agriculture or statecraft, or such with special purposes, like providing help for writing poems or quoting classical statements (Peiwen yunfu 佩文韻府, Pianzi leibian 駢字類編 or Zishi jinghua 子史精華), or for the preparation to the state examinations. Quite interesting are the richly illustrated encylcopaedias Sancai tuhui 三才圖會 and Tushubian 圖書編.
The main difference between Western and Chinese encyclopaedias is that leishu to not feature definitions of lemmata, but assemble quotations from original sources, often such of "classical" character (diangu 典故), flowery language (cizao 辭藻, hualici 華麗詞) or couplets (pianyu 駢語,). The translation "reference book" is therefore quite appropriate. For this reason, the early leishu are themselves important sources for the collation of old texts (see jiaokanji 校勘記) or the collection of fragments of lost books. The Taiping yulan 太平御覽, for instance, quotes from no less than 2,579 books, of which eighty per cent are lost (Fang 1990: 189).
Most of the important leishu are arranged according to themes (lei 類) that were in the course of time more or less standardized. They usually begin with astronomy and geography, proceed to a wide range of themes in politics and administration, then go over to human matters and daily life, and end with animals and plants. In this way, encyclopaedias also reflect hierarchy in the universe. In most encyclopaedias, the themes are arranged in main topics and sub-topics. Yet some leishu are arranged according to rhymes of lemmata (Peiwen yunfu, Yongle dadian 永樂大典).
The term leishu is first used in the bibliographic chapter (ch. 57-60 Yiwenzhi 藝文志) of the official dynastic history Xintangshu 新唐書 and the catalogue Chongwen zongmu 崇文總目 from the early Song period 宋 (960-1279).
The oldest known leishu was the Huanglan 皇覽, compiled by Liu Shao 劉劭 (c. 168/171/182-c. 245) and Wang Xiang 王象 during the Three Empires period 三國 (220~280 CE). The structure of the more than 600-juan long Huanglan was imitated by several early encyclopaedias like Liu Yao's 劉杳 (487-536) Shouguang shuyuan 壽光書苑, Liu Xiaobiao's 劉孝標 (462-521) Leiyuan 類苑, Xu Mianling's 徐勉領 Hualin bianlüeXiuwendian yulan 修文殿御覽 from the Northern Qi period 北齊 (550-577) (all lost).
The earliest surviving encyclopaedia is Yu Shinan's 虞世南 (558-638) Beitang shuchao 北堂書鈔. Three other encyclopaedias from the Tang period 唐 (618-907) are Ouyang Xun's 歐陽詢 (557-641) Yiwen leiju 藝文類聚, Xu Jian's 徐堅 (659-729) Chuxueji 初學記, and Bai Juyi's 白居易 (772-846) Baishi liutie 白氏六帖 (during the Southern Song period combined with Kong Chuan's 孔傳, 1065-1139, Kongshi liutie 孔氏六帖 to the book Bai-Kong liutie 白孔六帖).
It was particularly the book Yiwen leiju which set the standards for all later encyclopaedias according to the principle that "subjects (lemmata) are presented first, and then several [suitable quotations from] literature" (shi ju qi qian, wen lie yu hou 事居其前，文列於後).
Emperor Taizong 宋太宗 (r. 976-997) and his successors of the Song dynasty decreed the compilation of several encyclopaedias which would assemble the knowledge of the day and so represent the cultural and political world of the Song empire. The leishu of that time were known as the "four great books" (Song si da shuTaiping yulan (a general leishu), Taiping guangji 太平廣記 (specializing on unexpected or extraordinary things), and Wenyuan yinghua 文苑英華 (which is actually not a leishu, but an anthology of literature, but arranged according to genres, i.e. lei), all compiled under the supervision of Li Fang 李昉 (925-996), and Cefu yuangui 冊府元龜, an encyclopaedia specializing on politics, compiled and edited by Wang Qinruo 王欽若 (962-1025) and Yang Yi 楊億 (974-1020).
The most important Southern Song period 南宋 (1127-1279) encyclopaedia is Wang Yinglin's 王應麟 (1223-1296) Yuhai 玉海, of which only prints of lower quality were made. Another reason why the Yuhai is rarely used is that the arrangement of sources deviates from that of all other standard encyclopaedias.
The largest premodern Chinese encyclopaedia was compiled on imperial decree of the Yongle Emperor 永樂 (r. 1402-1424), and was therefore called Yongle dadian. It was never printed, but only existed in a few handwritten copies, of which today only a small part is preserved. While most leishu give only reference by quoting one or several phrases from older sources, the Yongle dadian quoted whole books (and thus preserved texts never published or preserved anywhere else), and was therefore widely used for the compilation of the imperial series Siku quanshu 四庫全書. The compilers extracted no less than 500 more or less complete books from the Yongle dadian (Fang 1990: 189).
The Kangxi Emperor 康熙 (r. 1661-1722) of Qing dynasty 清 (1644-1911) entrusted Chen Menglei 陳夢雷 (1650-1741) with the supervision of their "dynastic" leishu project, the Gujin tushu jicheng 古今圖書集成, which served for one and a half centuries as an important sourcebook on various matters concerned with China.
Apart from these government-sponsored (guanxiu 官修) publications, there is a wide range of privately compiled encyclopaedias.
The largest descriptive overview of Chinese encyclopaedias is found in Zhao Hankun (2005), whose handbook also includes leishu which are long lost. Of more than 600 leishu books that were written in imperial times, 316 survive (Fang 1990: 189).