Boston-based Boundless, which has become a darling of the open educational resources movement seen as threatening traditional textbook publishers, offered versions of textbooks that would normally cost scores if not hundreds of dollars. It pitched what it offered as “textbook replacement,” created by essentially reverse engineering popular textbooks. Boundless attracted considerable attention, including an $8 million round of venture capital funding led by Venrock, an investment group started by the Rockefellers. (Boundless currently generates no revenue, its co-founder and CEO, Ariel Diaz, said Thursday.)