You are not required to inform anyone–author or publisher–if, or how, you’re using an open textbook. The open-copyright licence with which an open textbook is released states that you are free to reuse, retain, and redistribute it as you wish (as well as revise and remix it).
However, if you’re an instructor with a post-secondary institution in British Columbia, please fill out the BCcampus Open Education Adoption Form so that we may track statistics on usage in this province.