I think I heard somewhere that if one is writing a book on the subject, you can’t take chapters and send them into magazines as articles. But, I don’t know if that’s true, and don’t know the reasoning behind it.
Publishing excerpts from your book is a great advertising vehicle, especially if those excerpts are published in well-known publications! There are also sites that allow you to post excerpts, but not for any compensation, of course. I published an article on this topic last year, complete with links for sites that accept book excerpts. See: ONLINE BOOK MARKETING THAT WORKS PART II: Where to Distribute Free Book Excerpts