The book itself is a series of 15 of Williams' paintings that tells the story of a hare who loses the gift he's transporting from the moon to the sun which becomes the object of the hunt.
The finding of the hare has become mired in controversy - see here - and the actual solution to the puzzle was spectacularly convoluted. Below is an extract from the Wikipedia page (that the above link takes you to) because I'm not even going to try and explain it...
The solution to the Masquerade puzzle is elaborate. In each painting, lines should be drawn from each creature's left eye through the longest digit on their left hand to a letter in the border. Then from left eye through the longest digit on their left foot, right eye through the longest digit on their right hand and finally right eye through the longest digit on their right foot. This is only done for any eyes that are visible in the drawing. The resulting letters form individual words, revealed either by anagramming or by applying the order hinted at by the Sir Isaac Newton painting, in which all of the creatures of the book are represented as puppets hanging in a line from left to right.
Decoding and following this method reveals the nineteen-word message:
CATHERINE’S LONG FINGER OVER SHADOWS EARTH BURIED YELLOW AMULET MIDDAY POINTS THE HOUR IN LIGHT OF EQUINOX LOOK YOUTaking the first letter indicated by each painting, the acrostic “CLOSE BY AMPTHILL” is revealed. Properly interpreted, the message told one to dig near the cross-shaped monument to Catherine of Aragon in Ampthill Park, at the precise spot touched by the tip of the monument’s shadow at the stroke of noon on the date of either the vernal or autumnal equinox.
Many additional hints and "confirmers" are scattered throughout the book. For example, in the painting depicting the Sun and the Moon dancing around the Earth, the hands of the two figures are clasped together, pointing at the date of the spring equinox.
Hope that didn't make your head hurt too much.
Williams' experience of the Masquerade phenomena caused him to distance himself from the public eye but he continues to produce work including a number of major pieces of public art including Cheltenham's 'Wishing Fish Clock' and the Dragonfly maze in Bourton-on-the-Water both situated in the county where he maintains his studio.
He remains a fascinating and unique artist utterly and triumphantly out of step with modern art trends who has spent his life creating a body of work that reflects himself and the world as he sees / wishes it.