A miniature book is a book that is usually not more than three inches in width, thickness, or height, however, this is mostly true in the United States. Outside of the USA, books up to four inches wide or tall are considered miniature. 

Origins:

The answer of where mini books originated will vary depending on who you talk to. Some say they started in the middle ages. Others say they started with Sumerian clay tablets using the cuneiform script, dated back as far as 2500 B.C. It remains a bit of a mystery, and depends on what some may consider a book.

Usage:

Originally mini books were said to be made for convenience, and, as quoted from MBS (the Miniature Book Society), they consisted of "large proclamations transcribed into miniature for ease of storage; miniature bibles for monks to carry tucked in their pockets; miniature books of etiquette for young Victorian ladies to discreetly reference for proper conduct. Queen Mary made them very popular when, in 1922, 200 miniature books were produced for display in the library of her miniature doll house."

Common Collecting:

The most common reason for collecting miniature books today isn't usually to read them! Rather, it is to examine the craftsmanship and the skill used in producing the books. It could be anything from delicately threaded pages to small creases for a pop up to even small vampire kits, complete with little bottles of potions and chains to open the case. Miniature books are more currently regarded as works of art.