Whether you have a homegrown course "packet" or aspire to create an entire textbook, Rod Library can support you in creating, licensing, and sharing materials that you intend to be free of charge. This includes modifying existing materials that are openly-licensed. Details: https://guides.lib.uni.edu/oer/authoring
For all natural sciences open textbooks in Open Textbook Library, see https://open.umn.edu/opentextbooks/subjects/natural-sciences
Subcollections are available for Biology, Chemistry, Earth Sciences, and Physics. Be sure to evaluate each resource for quality and usage rights.
For more, use the Find Open Textbooks tab on the left side of this guide. Rod Library's OneSearch also includes some open textbooks as well as licensed ebooks; note that if you plan to use a library-licensed ebook for a course, you need to notify your liaison librarian well in advance (more on textbooks & library ebooks).