Open-source textbook firm Flat World Knowledge has raised an additional $2.5 million in funding. The company said back in March that it had raised $8 million in a first round of funding—but a spokesman says that round was expanded because of “momentum” in its business. The startup says that this fall more than 40,000 college students at 400 colleges used its textbooks, which it makes available for free online and are registered under a Creative Commons open license, up from 1,000 students at 30 colleges last spring. The additional funding comes as there appears to be a growing movement among students and colleges to find alternatives to purchasing high-priced textbooks. Rental textbook firms Chegg and BookRenter have both raised their own rounds in recent months.