I've got 3 going at the moment:
[Physical]
The Hitchhiker's Guide To The Galaxy Ultimate Collection - Douglas Adams
Technically this is 6 books back to back - it's the whole series. These are some of my favorite books of all time, and I make sure to re-read them every few years to keep them fresh in my mind. I would recommend this series to everyone.
The Andromeda Strain - Michael Crichton
Another one I've read previously and love. I read this many years ago and I've been craving some Crichton again recently so I decided to start back here. I recommend this to anyone who likes a good, science-based thriller.
[Audio]
Nuking The Moon - Vince Houghton
This is an interesting book so far, but it's not a favorite. I'm not super fond of the author's writing style. It's got a bit longer to impress me before I put it down and move onto something else, though. I can't recommend it yet - we'll see.