Unsolved mysteries of science
This is a book in currently reading... heck, I'll post my review in Amazon (I finished it a second ago):
This book is just not about what science is struggling with right now. Every chapter starts with a rather comprehensive introduction on each of the subjects this book deals with like black holes, language, evolution, etc. This gives a good background to understand how the current theory or theories evolved and why they are not complete. It also makes you an informed individual of the XXI century.
Although it dates from the year 2001, after reading this book your knowledge of science will be quite up to date, at least from the layman's perspective. Just a few days ago I've stumbled upon two articles that showed me how the controversies depicted in this book are quite alive and kicking. "The interpreter" (The New Yorker magazine) is a perfect continuation to the "How do we learn language?" chapter, where Chomskyian rather-unchallenged-until-recently theories are covered.
"Horizontal and Vertical: the evolution of evolution," an article published in the New Scientist a week ago, discusses the recent evidence for horizontal gene transfer. This theory is part of the second chapter.
Another reviewer says it's a rather symplistic book, with which I completely disagree. I might be so if you read scientific magazines every day, but for me (an electronics engineer) it was an entertaining way to learn about several different fields of science. Although I have to concede that you won't receive a Ph.D. after finishing this book.