What is the single most influential book every beer enthusiast should read? If you could go back in time and tell yourself to read a specific book about beer, which book would it be?
Tasting Beer by Randy Mosher is by far the most valuable beer book I've ever read.  It covers the styles you'll encounter, the flavors, the technology, the history, the science, and more.  I was a fairly knowledgeable beer enthusiast when I read it, but I wish I'd read it ten years earlier.  If I had, it would have increased my enjoyment of getting to know beer ten-fold by increasing my  understanding of what I was drinking.