I'd suggest you first read Phantoms in the Brain by V.S. Ramachandran. He explores some pretty amazing quirks about the human brain.

Then, if you're still interested, read The Selfish Gene by Richard Dawkins. It's a pretty good overview of biological principles and true evolution.

I had watched V.S. Ramachandran's talk about the Brain on TED as was fascinated and I recently read Dawkins The God Delusion and find it to be very interesting and easy to read.

If the books suggested by you and authored by those two are easy to grasp as their talk/other book, I would definitely buy them.

I don't anything about medicine, but it seems that Gray's Anatomy (http://en.wikipedia.org/wiki/Gray%27s_Anatomy) is one of the classic books.

The edition of 1918 is in public domain an available online: http://www.bartleby.com/107/ and the illustrations are simply gorgeous.

Gray's Anatomy is a textbook, and probably isn't a great casual read.

It's fascinating if you're already interested in biology and the human body, though.