VINCENT VAN GOGH: THE LOST ARLES SKETCHBOOK By Bogomila Welsh-Ovcharov Abrams/Editions du Seuil, $85, 288 pages We occasionally hear about the discovery of a previously unknown artifact, such as a painting or musical composition. This revelation tends to lead to vigorous debates, disagreements and denials from experts in a particular field. Which brings us to Bogomila…