Mr. Penumbra's 24-Hour Book Store by Robin Sloan
I had someone recommend this book as "geeky and fun" because of it's references to various computer programs. This book, however, is so much more that that.
This book combines Old Knowledge and New Knowledge and weaves them together with a wonderful mystery - several actually. I hesitate to say much more because I don't want to accidentally drop any spoilers. I will say this, you should read this book. It is a fun and entertaining escape.