The Unfolding of Language: An Evolutionary Tour of Mankind's Greatest Invention by Guy Deutscher
I would suggest that if you are going to read, and only one, book about linguistics, this would be the book to read! Don't let the evolution aspect put you off. Guy Deutscher's goal is to speculate about language change in the past based on the language changes we see happening around us today, every day. It's exciting stuff, and a very fun read, written with a general audience in mind.