This was written by my undergrad advisor and his PhD student (who is now also a prof at BYU). I think it was partially funded originally by an exploratory grant from the NSF. The website has editions back to 2008, but there were earlier ones--we used it when I took Optics in 2007. Students were involved in developing the material, by contributing to the historical profiles, participating in filmed demonstrations (also on the website), and of course providing feedback.<p>A nice thing about having an open textbook like this is that I can refer to it anywhere, instead of only at the office where my other texts are.
tldr: This is notable because it's a high quality textbook being made available free to students, universities, and anyone else. Contrast this with what many see as an inefficient commercial market for texts that are horribly expensive yet usually don't make the authors rich.<p>The restrictions are minimal. Authors retain copyright, ask that no one profits from reselling, and that you don't post answers to the problems online. Betting pool open on how long the latter will last.