Fore-edge painting

A fore-edge painting is a scene painted on the edges of book pages. There are two basic forms, including paintings on fanned edges and closed edges. For the first type, the book's leaves must be fanned, exposing the pages' edges for the picture to become visible. For the second closed type, the image is visible only while the book is closed.
The fundamental difference between the two fore-edge styles is that a painting on the closed edge is painted directly on the book's surface (the fore-edge being the opposite of the spine side). In contrast, the fanned fore-edge style has watercolor applied to the top or bottom margin (recto or verso) of the page/leaf and not to the actual "fore"-edge itself.
Variations.
A single fore-edge painting includes a painting on only one side of the book page edges. Generally, gilt or marbling is applied by the bookbinder after the painting has dried to make the painting completely invisible until the pages are fanned.
A double fore-edge painting has paintings on both sides of the page margin so that one painting is visible when the leaves are fanned one way, and the other is visible when the leaves are fanned the other way.
A triple fore-edge painting has, in addition to paintings on the edges, a third painting applied directly to the edges (in lieu of gilt or marbling). Edge paintings that are continuous scenes wrapped around more than one edge are called a panoramic fore-edge painting. These are sometimes called a 'triple edge painting.'
A split double painting has two different illustrations, one on either side of the book's center. When the book is laid open in the center, one illustration is seen on the edges of the first half of the book, and another illustration is on the edge of the second half of the book.
There are even examples of rare variations that require the book's pages to be pinched or tented in a certain way to see the image.
History.
The earliest fore-edge paintings date as far back as the 10th century; these earliest paintings were symbolic designs. Early English fore-edge paintings, believed to date to the 14th century, presented heraldic designs in gold and other colors. The first known example of a "disappearing" fore-edge painting (a painting not visible when the book is closed) dates back to 1649, while the earliest signed and dated fore-edge painting dates to 1653: a family coat of arms painted on a 1651 Bible.
A legend regarding how hidden fore-edge painting on books first began states that a duchess and friend of Charles II of England would often borrow his books, sometimes forgetting to return them. As a result, the king commissioned the court painter, Sir Peter Lely, and the court bookbinder, Samuel Mearne, to devise a secret method to identify his books. They worked out a plan to paint a hidden image on the edges. When the king visited the duchess, he spotted a familiar-looking book on a shelf. As he was leaving, he took the book from the shelf to reclaim. The duchess protested, but the king fanned out the pages of the book to reveal the royal coat of arms. 
Around 1750, the subject matter of fore-edge paintings changed from simply decorative or heraldic designs to landscapes, portraits, and religious scenes usually painted in full color. Modern fore-edge painted scenes have many more variations as they can depict numerous subjects not found on earlier specimens. These include erotic scenes, or they might involve scenes from novels (like Jules Verne, Sherlock Holmes or Dickens, etc.). In many cases, the chosen scene will depict a subject related to the book, but in other cases, it did not. In one instance, the same New Brunswick landscape was applied to both a Bible and a collection of poetry and plays. The artist, bookseller, or owner decides on the scene; thus, the variety is wide.
The technique was popularized in the 18th century by John Brindley (1732 - 1756), publisher, and bookbinder to the prince of Wales. and Edwards of Halifax, a distinguished family of bookbinders and booksellers.
The majority of extant examples of fore-edge painting date to the late 19th and early 20th centuries on reproductions of books originally published in the early 19th century. 
Contemporary Fore-edge Paintings.
Fore-edge painting as a craft is deemed critically endangered in the contemporary era. The Heritage Crafts Association (HCA) states that there are currently only three professional artists that practice this medium: Martin Frost, Margaret Allport Costa, and Clare Brooksbank. 
The remaining artists that practice fore-edge painting are amateurs and leisure makers numbering fewer than sixty. According to the HCA, there are currently no formal trainees in the art form.
Martin Frost is currently the only professional full-time fore-edge artist. He has created over 3,500 fore-edge paintings since he started his career in the 1970s. In 2019 he was presented with the MBE in the New Year Honours list by Queen Elizabeth II. According to a profile he did with the BBC, his training was in theater, where he painted backdrops for plays until he was introduced to this craft by a friend who happened to be a fore-edge painter. 