CK-12 is a website that provides free textbooks that can be fully edited and are copyrighted such that you can republish them. For me, that means I can distribute them to my Homeschool Connections students. The copyright laws for online classes are far more strict than they are for face-to-face classes, including specific language about educational institutions needing to be accredited. (Homeschool Connections offers a variety of courses to supplement homeschoolers rather than a comprehensive curriculum so we really have no urgent need for accreditation.)
I've referenced the site for many years since they mostly have math and science content, though they do have English, History, and SAT prep as well. Years ago I planned to use their ebooks and edit them for my anatomy and physiology courses, but I soon found I was editing more than I was keeping and so I wrote my own material using mostly Wikimedia to find images that I could republish.
I forgot about the site until I recently put together a middle school life science course. Trying to decide what topics I should cover and how complex I needed to be (or not), I visited CK-12 again. The site has been completely transformed and made into something far more useful than what it was. Before it offered only complete books for entire courses, and while those are still available they have now broken everything down into topics. They have and extensive list of Life Science topics geared towards middle school (the Biology link has topics at the high school level.)
Using the site I could easily identify, assemble, download, and distribute ebooks for my class. I could edit them as well, though I would not say that is an easy process. Yes, everything can be edited, from the title to the images to the text itself but you have to do it all online at the site. Often something will look properly formatted on the site but appears quite different in the pdf--this is especially true if you edit images. Online they appear before a paragraph while in the pdf they are after and I have yet to figure out what I need to do to adjust it. Another drawback is that editing is very slow during high-volume times, typically school day mornings. The third problem is that you supposedly are able to output the final product in 3 formats--pdf, mobi (for Kindle) and ePub (for iPad and Android.) I have no problem getting the pdf shortly after requesting it (you are emailed a link from which you download the finished product) but I never receive links for the other two versions, ever.
Still, overall it has made putting customized science material together very easy. When it comes to creating Life Science materials for a Catholic homeschool audience, customization is critical in some areas, and that is what made CK-12 so helpful. Here is an example of the ebook I distributed for the first class.