The Friar and the Cipher is about a lot of things. It's about something called the Voynich manuscript, an old book found on a bookscouting tour in the early 20th Century by a Wilfred Voynich. The book is written entirely in a cipher no one has yet been able to definitively crack, and illustrated will all manner of pictures of plants, star maps, and naked ladies. The last 50 pages are about the various attempts to crack the cipher over the last 100 years.
The person who seems to be the most popular choice as author of the Voynich manuscript is Roger Bacon, a 13th century friar and scientist. So most of the book is about Roger Bacon. His life, his studies at Oxford, his feeling that the Church should embrace the scientific method of hypothesis and experiment as a way to better understand the universe , and thus, God, the lean years at Paris University where he was kept under lock and key and close observation by the Dominican friars and such who were running things. Because of the role the Catholic Church plays in the proceedings, the Goldstones also spend a lot of time on what was going on with the Church, the formation of the Dominican and Franciscan orders, their struggles against Frederick II, the reaction against scientific progress. And, since St. Thomas Aquinas was the proponent of a rival approach towards science and learning, he gets a significant amount of pages. And so does John Dee, since he was apparently obsessed with Roger Bacon a few centuries later.
All in all, it creates the feeling of a book where the authors didn't have quite enough on any of the subjects they really wanted to discuss, so they mashed it together, along with some other, ancillary information in a general attempt to give a sense of the wider setting affecting the characters. It isn't a bad idea, but if Roger Bacon is the subject of the book, then they probably shouldn't spend so much time talking about all these other things. The stuff about Dee sort of fits as someone who collected as many rare books as he could and shared them, helping to reintroduce Bacon (and other's) ideas to people who'd never heard of them, and he also may have owned the Voynich manuscript. But he wasn't much of a scientist, and the Goldstones don't really do a lot tracing the ownership history of the manuscript.
'After trying cribs, substitutions, concordances, frequencies, combinations, after checking provenance, medical history, philology, botany, and astronomy, after subjecting the cipher to both computer analysis and human instinct, no one was able to (persuasively) penetrate the code. There was too much detail in the manuscript, too much order, too much precision, for it to be a fraud, but too much obscurity to provide an answer.'