There is a popular kind of "mystery" novel in which the story becomes increasingly complex, with more characters, more possibilities, and more paranoid plots introduced until the reader gets swamped with them all. Then in the last few pages, the secret is unlocked, and something that could not possibly have been guessed is revealed as the key. Ex Libris repeats this formula in 17th-century England, and I found it as exasperating as the versions that take place in present-day California.
There are some nice historical touches in the book, although the narrator is clearly unrealistic in a number of respects in order to help the modern reader through the 17th century. The problem is that the plot moves slowly, the protagonist wanders around aimlessly for much of the book, and the digressions and explanations dilute any excitement one might have felt. The ending is simply absurd, and the reader feels cheated, as events that were never mentioned previously turn out to be key to explaining the mystery.
It was an excellent idea to attempt to import the modern noir thriller to this historical setting, but the pacing and plotting of the novel do not live up to the initial conception. I found that I had to force myself to finish the book.