The story
From red + book, as a fixed title for various official books from the 15th century, especially the Red Book of the Exchequer, a manuscript volume of the 13th century (OED).
Etymology from Wiktionary, CC BY-SA 4.0.
red book
Today it means: A book, often bound with a red cover, or online equivalent, that is an official, or canonical, collection of data, regulations, or writings. Full definition →