The Friar and the Cipher: Roger Bacon and the Unsolved Mystery of the Most Unusual Manuscript in the World has the longest title of all the books I've read so far. I'm glad to finish it, since now the "currently reading" box on the website won't be so stretched out.
This book reminded me a lot of Descartes' Notebook, for obvious reasons. The subject in both titles is a mysterious manuscript written in code or cipher by a medieval scientist who feared oppression by the church.
In both books, the authors' approach was similar. Introduce the medieval manuscript, then launch into a biography of the manuscript's author so that we understand the context in which it was written, then return to reveal what is known of the manuscript today.
Unlike the Descartes notebook, the book in The Friar and the Cipher exists today and is still being researched and studied. The Voynich Manuscript, named after the English bookdealer who discovered it in 1912, has defied scholars for more than 400 years. The authors recount fascinating attempts to decode the book ranging from World War II code breakers to the NSA and super computers. All have ultimately failed, and the attempts to read the book continue to this day.
There are undoubtedly better biographies of Roger Bacon, and better books on the Voynich manuscript, but this book provides an excellent introduction to both subjects. The writing is very clear and enjoyable, and manages to cover an astounding range of topics along the way.