Using a Creative Commons license is the easiest way to ensure that your text can be used by instructors around the world.
Considerations (excerpted from Considerations for licensors and licensees)
Remember the license may not be revoked.
Once you apply a CC license to your material, anyone who receives it may rely on that license for as long as the material is protected by copyright and similar rights, even if you later stop distributing it.
Think about how you want the material to be used.
Consider what you hope to achieve by sharing your work when determining which of the six CC licenses to apply. For example, if you want it to appear in a Wikipedia article, it must be licensed using BY-SA or a compatible license.
Make sure the license grants permission for what you want to do.
There are six different CC licenses. Two of the licenses prohibit the sharing of adaptations (BY-ND, BY-NC-ND); three prohibit commercial uses (BY-NC, BY-NC-ND, BY-NC-SA), and two require adaptations be licensed under the same license (BY-SA, BY-NC-SA).
Pay attention to what exactly is being licensed.
Consider clearing rights if you are concerned.
The license does not contain a warranty, so if you think there may be third party rights in the material, you may want to clear those rights in advance.
Some uses of licensed material do not require permission under the license.
If the use you want to make of a work falls within an exception or limitation to copyright or similar rights, you may do so. Those uses are unregulated by the license.