Historians are rarely satisfied with their evidence: They want more. When writing my political life of Abraham Lincoln, I lamented that the Civil War president didn't keep a private journal. Now, as the head of one of Oxford University's historic colleges, I have another fancy: to identify the anonymous 12th-century Jewish traveler whose Hebrew prayer book, quite possibly the oldest extant in Europe, is one of the many treasures in my college's library. Although this particular jewel has been in our possession for centuries, it has only now become the subject of scholarly scrutiny and excitement.

Corpus...