If you're interested in giving permission for free online editions, either now or in the future, write to email@example.com. Authors can donate their works to the public in a variety of ways, ranging from putting the work into the public domain to retaining copyright and donating a license. The Creative Commons has a page on approaches that one can use.
Many books have already been scanned in mass digitization projects, so making your book readable online may be as simple as granting permission to a suitable project. For instance, if you wrote any of the millions of books with copies now in Hathi Trust, you can simply complete and send a permission agreement authorizing them to make your book readable online.