The Complete Book with the Answer Explained
Which is too bad, because Williams' lush paintings deserve to be seen in a large hardcover version, not the tiny paperback available now. Not only are the illustrations beautiful to look at, but they are filled with details that invite the eye to search for clues. The text is also filled with riddles that are fun to solve too. This is a book you'll want to read again and again, even if you can't solve the main riddle.
One last point--the recent book "The Merlin Mystery" tries to duplicate Masquerade's feat, but the illustrator is no Kit Williams. If you want to read other puzzle books like "Masquerade", I recommend "The Egyptian Jukebox" by Nick Bartcock and "The Eleventh Hour" by Graeme Base. Neither of these books, I should warn, hide any treasure however.