There are websites such as Scribd that scan and make textbooks available on the world wide web. Scribd is a social publishing site, where tens of millions of people share original writings and documents. They do not ask the author's permission to put their books online. For authors who write for royalties from the amount of books they have sold, this means they are losing money. This is not just a concern for Eduational Publishing but for fiction as well.
These sites eventually remove unlicensed content from the web but they have to be petitioned to do so. There is a Copyright Infringement Takedown Notification on the Scribd website and they provide a Takedown Notification Template for authors.
But, surely they should have not put the books available for free download on line in the first place. They should be the ones seeking permissions and paying for licenses not teh authors having to fight for the right to get paid for their hard work.
Advice from the Society of Authors is to be vigiliant and to search for titles online on a regualr basis. If you find anything suspicious it should be reported to the publishers. The Publishers Association has set up a Piracy Portal to share information about copyright infringement. There is also a Copyright Infringement Portal, which targets websites offering infringing copies for free download, and will soon evolve to also target peer-to-peer sharing via torrents.
So check today. See if you work is being pirated.