The hallmark of an open education resource is the freedom you have to create, remix, and adapt it to your needs. Creating an OER does not necessarily require you to write an entire textbook or develop a complete learning object from start to finish - although you can certainly do that! - rather, the open education community is based not only on creation, but adaptation and improvement on what's already out there.
In fact, the various open licenses in use expressly encourage instructors to modify open educational resources to tailor them to their classes, students, and specific pedagogical needs.
Pressbooks is simple book production software. It allows you to write a book using the sofware or import a manuscript, choose a book design theme, and export into all the file formats you need to publish your book:
Pressbooks is used by authors and publishers around the world (Pressbooks, 2017, para. 1).
For online tutorials that teach you how to use Pressbooks, visit the following YouTube channel: https://www.youtube.com/user/pressbooks