Hundreds of years after a British book collector passed away, his volume of 66 luscious watercolors of orchard fruits has taken on a new life. The charming illustrations have been reproduced in facsimile in “The Tradescants’ Orchard: The Mystery of a Seventeenth-Century Painted Fruit Book” (Bodleian). Here and there, the unknown artist painted a bird, a frog, an insect, even a miniature squirrel. Were the pictures created merely for pleasure or as a catalog for a traveling salesman? The co-authors, plant scientist Barrie Juniper and art historian Hanneke Grootenboer, suggest that a new generation of detectives take up the mystery that has bedeviled them.