The conversation around eddies in the space-time continuum is the single funniest thing I've ever read. I haven't read those books for years but I still chuckle every time I am reminded of it for any reason. Douglas Adams was a true master of his craft.
Absolutely HHGTTG for me. Not the "best" books I've ever ready but easily the ones I can re-read the most and still get a laugh or discover something new (or something I at least forgot and get to re-enjoy).
The hitchhiker's guide to the galaxy.
There are many better books I've read, but I grew up with these, and would want access to copies at all times in my life.
Now... I'm off to make a Perfectly Normal Beast sandwich and ponder why Eddie is in the space time continuum.