Counterfeits are actually a very big issue in the textbook industry. Anyone who deals with used textbooks knows this and they need to be very careful to avoid legal action from publishers. Do a google image search on "counterfeit textbook examples" and you'll see how many of the counterfeits look pretty similar to the authentic books (often it's small things like coloring that give it away).

Obviously textbooks get expensive, but the thing that makes textbook counterfeiting especially successful is that most consumers who get cheap counterfeits don't care. The content is usually the same and it's not a status symbol like an authentic Gucci bag. So as a consumer, there's really not much difference to you, especially if you got the book for cheap.

Typically, counterfeits are lower quality, but someone who works in the industry once told me that he's seen counterfeits with higher quality printing than the originals, which makes the real books look like fakes!