I thought that this book by Steven Pinker was very interesting. It goes from sentence diagramming (which I don't think I ever learned) to the makeup of the brain to case studies of linguistic savants and computer programs that try to emulate human conversation. Though the information is densely packed into this volume, it is still entertaining. I wouldn't pick it up if I wanted a light read, but if you're as interested in linguistics as I am, this seems like it was a great introductory book.
Pinker's assertion is that humans are born with an instinct to learn language, and a specific window in which we are optimally prepared to learn it-- which is why people who learn second languages later in life learn more slowly, and have thicker accents. He also brings up the sad cases of those poor folk who have been raised without language, in insanely neglectful and abusive households. They can never learn any language as well as someone who is raised with it, though some have invented languages of their own.
Another particularly interesting aspect of this book was Pinker's propensity for bringing in examples from other languages. I liked it when he used examples from Japanese, since I've studied it, but there were some absolutely fascinating examples from various African languages as well. It's wonderful to see these wildly different languages broken down into their component pieces to find out that they are not so different after all.
I would definitely suggest that anyone interested in linguistics read this book-- I am sure I have not done it justice.