Non-fiction works are often poorly written, but there are some standout exceptions. These are the best non-fiction communicators I've come across -- people with a writing style that's pleasuable to read and gets the points across clearly. That is, good examples to learn things from.
Of course, I can only comment on those who I've read (which is substantially more than just these people, and does range beyond the subject-areas they represent :-)).
Paul Graham. Graham has got a pretty amazing style. What makes it stand out is his ability to draw out non-obvious conclusions from simple premises and to present them with real impact.
Richard Dawkins. Very good at clearly and simply communicating ideas and concepts, and a very clean, elegant writing style. From memory, I think The Blind Watchmaker is probably his best book from a communication perspective.
Steve Grand. Explains things well and a pleasure to read. Growing Up With Lucy, the second of his two books, is probably the best one to look at.
Clay Shirky. Also quite good.
Some other people who are pretty good: Daniel Dennett, Steven Pinker, Edward O Wilson.
The following concern some specific aspects of communication.
Jorn Barger. Pretty good at writing very short, one-line descriptions for his blog links.
Dennis Dutton. Quite good at writing one or two sentence descriptions for his blog links - in a way that entices the reader to check them out.
Jon Udell. Pretty good at presenting things that people might have a reservation about -- because it is new or different, for example -- in a way that'll make them less likely to feel that reservation. (I'm not sure this best captures things, I think I'd need to read some more of his stuff again...)
Scott McCloud. Very good at expressing ideas -- in comic book form. The idea that comic book form must somehow inherently involve superheroes doesn't make sense, and his book Understanding Comics he uses the form to convincingly demonstrate this with a non-fiction consideration of the properties of the form.