These are known as fore-edge paintings, and the practice began centuries ago. Fore-edge painting dates as far back as the 10th century, but 'disappearing' fore-edge painting only came into existence in about 1649. This means the paintings cannot be seen when the book is closed, and can only be seen if the pages are fanned slightly.
In order to paint these artworks on the edges of the paper, the edges of the book have to be clamped in the fanned position to make a flat surface to paint on. When the clamp is released, the image disappears. The pictures were painted first. Then gilt was applied afterwards, once the paint had dried. The gilt just masks the existence of the painting by covering the very edges of the pages.
Four books written by Robert Mudie, published in 1837 and titled 'Spring,' 'Summer,' 'Autumn' and 'Winter,' have hidden fore-edge paintings of the corresponding season hidden on their pages.
