Although not a new one, the concept for this book is a good one: newly discovered and possibly fraudulent information regarding the life of a dead artist. Frans Hals was not a painter well known to me, but Kernan did a good job of bringing Hals' life in 17th-century Haarlem, Netherlands into vivid detail.
However, unfortunately, the character who is charged with translating the Diaries is neither compelling, nor does he make any important discoveries about said diaries. The whole concept would have been much different if the diaries shed some light on an unknown part of the artist's life. The diaries simply contain the comings and goings of an interesting character who happened to be a famous artist.
For an easy read, this certainly hits the spot - especially if your interest lies in the art of the Dutch Masters or in the history of the Netherlands. Otherwise I would recommend "Possession" by A.S. Byatt for a book in this genre that is both better written and much more satisfying.