Textbooks prices are increasing and Jonathan Tomkin, associate director of the University of Illinois of Earth, Society and Environment, decided to give his students a break by writing a textbook of his own and offering it for free online.

Tomkin and two professors from the University of Illinois at Chicago and the University of Illinois at Springfield, combined their collective brain power to write the open-source textbook “Sustainability: A Comprehensive Edition.”

Students in need of a copy of “Sustainability: A Comprehensive Edition” are able to download a it for free from the Open Textbook Library and obtain a printed copy for only $25, which is a fraction of the cost of most other textbooks on the market.

“It’s hard to buy a textbook for less than $100 these days,” said Tomkin. “I’m a big proponent of open-source textbooks. I think the quality can be really high, and I think they’re great for students.”

ESE 200: Earth System an ENSU 310:Renewable & Alternative Energy are two of the current courses being taught at the University of Illinois by Tomkin using the 560 page textbook. The book covers a great many topics including climate change and environmental an resource economics.

The wide scope of material covered by Tomkin has made the textbook a hit as thousands of students have downloaded a copy and six universities are using it for their own courses. The fact that it’s free certainly doesn’t hurt either, though.

“We need to use the best practices in education by using technology as much as possible. I think electronic textbooks are part of that best practice. Part of being efficient is not making things cost more, I think we should be using technology to make things cost less,” says Tomkin according to USA Today College.

The development of Tomkin’s book is part the Open Source Textbook Initiative started at the University of Illinois in 2012 after the U.S. Department of Education awarded $150,000 in grant funds to “design, create and implement open-source educational materials for use in introductory college courses.”

This initiative comes at the right time as some colleges face tuition hikes, which stings enough without the added $1,200 bill students rack up on textbooks each year, according to the U.S. Public Interest Research Group study from 2014.