You're teaching a class from a book you didn't write. You have permission to distribute fabric requirements and instructions on how many cuts from the pieces of fabric, but that's about it. If you want to distribute more (i.e., step-by-step photographed instruction), you should really ask for permission.
Put yourself in the author's shoes if it makes it easier. If you wrote a pattern, would you like someone to be able to copy that pattern and make money without giving you credit or compensation? She went through a lot of work to get her book published. If someone buys one copy and teaches a class of 1,000 you can see how that dilutes her success.
Chances are, she'll grant you permission to do what you want, but it's always polite (and legal) to get permission.