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It is written for general audiences, but was enjoyable and easy to read. Another book I really enjoyed that is a great example of an expert communicating complex subject material clearly to more a more general audience is Brian Greene's Elegant Universe. It is a bit older, so the science may or may not be dated now.

Brian Greene is definitely a great communicator. I haven't read his book (for obvious reasons, I am more on the side of reading the specialized books on the topic, which is not an excuse for skipping the general audience ones), but I have heard a lot of good things about him.

Another thumb up here for Elegant Universe. It's a really great entry point for all physics, but with a special leaning towards string theory