Figure out what your audience might want to know about your subject, and tell them that.
If you make assertions about history, traditions, myths, etc., back them up.
Don't pad your material just to make a book. If it's really only an article, write it that way and submit it to a journal.
If you are writing about Wicca, put "Wicca" somewhere in the title. Non-Wiccan Pagans and Mages are very tired of being told they are just like Wiccans.
If you have personal rants to get off your chest, get a blog. Leave them out of your books.