Definition through Hewlett Foundation:
“Open Educational Resources are teaching, learning and research materials in any medium – digital or otherwise – that reside in the public domain or have been released under an open license that permits no-cost access, use, adaptation and redistribution by others with no or limited restrictions.”[4]
Citation: William & Flora Hewlett Foundation. “Open Educational Resources.” Accessed June 15, 2019. https://hewlett.org/strategy/open-educational-resources/

If a resource is not free or openly licensed, it cannot be described as an OER. For example, most materials accessed through your library’s subscriptions cannot be altered, remixed, or redistributed. These materials require special permission to use and therefore cannot be considered “open.” Table 1 below explains the difference between OER and other resources often misattributed as OER.

What are open textbooks? From BCcampus website: https://open.bccampus.ca/what-is-open-education/what-are-open-textbooks/

Open textbooks are a subset of open educational resources (OER) and reside in the public domain — where copyrights have been waived by the copyright holder or copyright has expired — or have been released by the copyright holder under an open-copyright licence. The B.C. Open Textbook Collection, Canada’s first major repository of these OER, is available for all to use.
Open textbooks are available digitally, accessed online or through shareable formats, to be freely used by anyone: students, instructors, librarians, and members of the public. In general, they can be modified, printed, shared, retained, remixed, and reused. If the work has been released with a Creative Commons – or other open-copyright – licence, the user must adhere to that licence’s legal requirements.

Searching for OER can be difficult when you’re starting from a narrow perspective. For the most results, start with a broad search focused on your discipline. Once you’ve brought together a large collection of resources, then you can begin to limit your results.

6 minute Video: https://pressbooks.bccampus.ca/uvicoerguidelines/chapter/finding-oer/

Now evaluate the OERs that you have found. There are a number of rubrics available to help you evaluate the resources. BCcampus has developed a set of criteria for evaluating OER. See if textbook/resource matches your criteria and based on content, presentation, online accessibility, production options, platform compatibility, delivery options, interactivity, consistency between online and printed versions, and available ancillary material (test banks, PowerPoints, etc.). You can read these reviews when you search for different books in the collection. If a book hasn’t been reviewed, consider submitting a review request.  The reviews are comprehensive.

Before you think about creating a NEW open textbook, consider adopting an open textbook that you found. Adopt the textbook and gather feedback about the textbook from your students. Adopting open educational resources means that you are selecting openly licensed free resources (like textbooks) to use “as is” in your classroom. For example, Physics faculty adopted a number of chapters from University Physics Volumes 1-3 published by OpenStax for their PHYS 120, 130, 216 courses. The Copyright and Scholarly Communications Office (CSCO) at UVic Libraries assisted the faculty by collating the chapters required for each course into a pdf for easy by students. The CSCO also curated the Chapter content on Pressbooks for the three courses. Earth and Ocean Sciences have adopted open textbooks for a number of their courses, e.g.: EOS 120 – Physical Geology. History 230A Canada to Confederation adoption an open textbook for their course as well.

“The term adaptation is commonly used to describe the process of making changes to an existing work. Though we can also replace “adapt” with revise, modify, alter, customize, or other synonym that describes the act of making a change.” From BCcampus Adaptation Guide (CC BY)

Reasons to Adapt an Open Textbook
One of the benefits of using an openly licensed textbook or other educational resource is that you are free to adapt it to fit your needs. In other words, you can adjust the educational resources to fit your course curriculum, not the other way around.  

https://opentextbc.ca/adaptopentextbook/chapter/choose-licence/

All OER are made available under some type of open license—a set of authorized permissions from the rights holder of a work for any and all users. The most popular of these licenses are Creative Commons (CC) licenses, customizable copyright licenses that allow others to reuse, adapt, and re-publish content with few or no restrictions. CC licenses allow creators to explain in plain language how their works can be used by others.