I would have to say "The Lake of Dead Languages" by Carol Goodman, I picked it up by chance and I fell in love with it. I've passed it on to a few other people and they've all enjoyed it as well.
Defining the Wind by Scott Huler is absolutely brilliant. It is a non fiction book in which the author becomes obsessed with the Beaufort Wind Scale when he comes across it while working as a dictionary editor. So he goes on an amazing search to learn about the history of the scale and the man whose name it bears. It discusses history, meteorology, literature, cartography, the list goes on. I was a history and meteorology student in college and this was the most amazing book I'd ever read. I've re-read it several times.
Ironically, shortly after I read it the first time I was at the oldest pub in New Orleans and started chatting with some gents I met there. They were visiting from the UK. They worked in the Hydrography Office (in charge of all military sea charts for the British navy). I was excited to tell them I had just read a book about the first two Head Rear Admirals of the UK Hydrography Office, Dalrymple and Beaufort. I proceeded to gush about what I'd learned from the book. They stared at me and after a minute of silence in which I thought "Oh God, I've put them on geek overload", they told me that I was sitting next to the CURRENT Head Rear Admiral. -MIND-BLOWN-
I have actually managed to stay kind-of in touch with this gentlemen and he has invited me to tour the office if I'm ever in the UK and he's show me the archives where Beaufort's original notebooks are and where they have Captain Cook's original sea charts of places like Hudson Bay and Botany Bay. If I ever make it to England I am so cashing in on that tour, and it's all thanks to an amazing undiscovered book.