This was an entertaining read that reminded me a bit of Cory Doctorow's Little Brother: although Robin Sloan's book is more of a literary mystery and less of a call to action against an increasingly intrusive government (OK, Penumbra isn't that at all), the attempts to reconcile old knowledge and new today strike a similar chord. As Sloan notes on his Web site, "This is a novel about books and technology, cryptography and conspiracy, friendship and love" -- much of which could also be said about Little Brother. You probably won't learn as much about Aldus Manutius, early printing, and font punches in that book, though.
robin sloan