Dictionary Day is celebrated every year in October — on October 16, to be precise, the anniversary of the birth of Noah Webster — to commemorate the organizational wonder that is the dictionary. Since Webster’s day, dictionaries have expanded way beyond basic collections of words to encompass all fields of study, and serve not only as immensely useful learning tools but also as powerful guides for arranging areas of thought and study. Here are a few classic examples of the specialized dictionary.

A Concise Etymological Dictionary of the English Language: This 1901 volume completely updates the classic reference work first published in 1882, providing a staggering number of words, including those most frequently used in everyday speech and those most prominent in literature. They appear along with their definitions, their language of origin, their roots, and their derivatives. Those who are fascinated with the English language will find much to explore here and many overlooked but interesting tidbits and treasures of an ever-evolving language.

Illustrated Bible Dictionary: “The fruit of many years of loving labor,” Scottish Presbyterian minister Matthew George Easton’s Illustrated Bible Dictionary has become a classic reference for those studying the Bible. Originally published in 1897, three years after Easton’s death, it contains almost 4,000 entries and dozens of illustrations and maps. Readers will find definitions of terms ranging from Alpha to Zuzims. In between they’ll find entries both obscure and common, such as Emims (a warlike tribe of giants), Hagar (Sarah’s handmaid), immortality, meekness, Pentecost (the feast of harvest), seventy weeks (a prophetic period of time before the coming of the Messiah), sling (what David used to slay the giant), and Zorah (Samson’s birthplace).

A Dictionary of Theosophy: Here, in one concise volume first published in 1926, is a glossary of the language of theosophy, or “the essential truth underlying all religious, ethical, philosophical, and other teaching,” from Abhava (”Non-being, non-existence, negation”) to Zarathushtra (”The name given to one of the Servers”). Steeped in the spirituality of ancient India and fueled by the occult fads of the early 20th century, this is a fascinating, highly browsable guide to a forerunner of today’s wide-ranging, metaphysically encompassing New Age thought.