When determining whether or not to use an open textbook (or other OER component), faculty members should follow the same evaluation process as they use for commercially-published textbooks and course components.
Open textbooks, courses, and components do sometimes present additional challenges, since some of them do not list the author(s) and/or publication year. However, if you are teaching in a discipline where the publication date of the material is less important than the content itself and how it is presented (e.g. algebra, history, writing), then you will find many potential open textbooks and other resources.
Another aspect of OER materials to keep in mind is licensing. This is especially important if you want to modify the materials that you find. For more details on licensing, see OER and Licenses.
Textbooks listed in these repositories are available in digital form for free, with no access restrictions.
In addition to Open Textbooks, many items of Open Courseware are also being developed and shared. These can include lesson plans, activities, worksheets, and videos; but entire courses are sometimes available for reuse and adaptation.
There are several major repositories of OER that contain mixtures of the different types of items available--textbooks, videos, reading lists, activities/worksheets, etc. If you are looking for an overview of what is available in a subject area, as opposed to looking specifically for a textbook or full course, then you should try searching these sites.