SUNY Geneseo's Milne Library has developed the Openly Available Sources Integrated Search (OASIS) tool to make finding things easier, and it searches more than 100 sources for OER content.
This is particularly true of many titles that would otherwise be out of print, but there is also a growing number of those initially published open. While these texts cannot be modified, they can definitely be assigned to students as part of course readings, and are available to read online and/or download for free.