Traditional publishers certainly see value in branding nonfiction books by series. Think of Wiley's "For Dummies" series, O'Reilly's "Missing Manual" series, Adams Media's "Everything Book" series. Readers seek out those series because they know the level (beginner to intermediate) and style (humorous) will be consistent from one book to the next. All of those publishers also give the titles similar-looking covers so readers can recognize series titles at a glance.
Thanks Nancy, especially for being specific to nonfiction. I will leave these as series, and see. I just wanted to make sure that series is not just for fiction as many of us think.
That's all I wanted. Wish you all the best.