I've read this book a few months ago and even though there are some nice parts in it, I felt like it didn't go deep enough in the subjects it raised. Some subjects were repeated many times and explained in depth while others, as important if not more, were talked about in a single paragraph: "This concept is very important to make your levels interesting, for example this is bad and this is good. The end." (Exaggerated of course)
Obviously you weren't aiming for a book of the size of the Encyclopædia Britannica, so it's understandable why you couldn't treat every subject with the same dedication, but still, I found some chapters of the book to be pretty deceiving, in that they did little more than citing some level-design related features and then passed on to something else. It was a good introduction to level-design, no doubt with that, but I was expecting to find something more. I might have been misled by the name though, and by the fact that I'm generally more interested in reading about gameplay than visuals and that the latter was more important than the former in your book.
Also, even though I can't say it was THAT disturbing, I have found it to contain a lot of typos, missing words etc. and even though these can also be found in the latest Stephen King best-seller, I have found these mistakes to be very regular in your book, and those that I knew MS Word would have pointed out for you ended up irritating me a little bit. I'm in no way a writer, but I read a lot and expect these mistakes to be limited when I buy a book of this kind (more expensive than my summer polar I mean.)
I'm sorry if my post seems like I hated your book: it's not the case. On the positive side, I think you raised some very interested points, such as the references to photo composition, which I had never read about in any level-design/game-design book I read. I just felt like pointing out the things that annoyed me in the book, but I still keep it as a whole in good esteem. I read that you were working on another one, so good luck with it!