Einstein's Dreams by Alan Lightman. I first came across this book when it was an assignment for a literature course in college. I enjoyed the book then, so when I was looking for a quick read recently, I picked it back up for a second time.
The book presents a large number of short chapters, 30 in total, each one a different dream of Albert Einstein's that revolves around a various theory of time. One chapter will tell how time passes more slowly the faster you are traveling, while another chapter will explore a theory about time travel.
I enjoyed this book and it's forcing me to expand my mind, but I could only take the chapters in small doses. After reading each chapter, I would think about the theory it presented for a few days before moving on to the next chapter. If you're interests lie in dreams, time travel, or the questions of the unknown, then you'll enjoy Einstein's Dreams.