I've had this situation twice over the years. In one case it was a local hotel publishing a guidebook that copied stuff lock stock and barrel. I contacted the hotel, they contacted the author who admitted what he had done and they removed the stuff from their website. The hotel representative in the meantime couldn't heap enough criticism on the work that they had copied and published. Given that she was so combative, it would have taken a court order to seize their inventory of printed materials.
In the other case it was the author of a book. The problem was not so much the printed material as the fact that in her infinite wisdom, the author decided that she also had the authority to give other people rights to republish our material that she had copied. So we shut all of those down, then while the lawyers were debating what to do, she got wind of it and pulled all the infringing material from her website.
In the end the lawyer said it would be worth trying to get an action against her publisher. But again, in the end I decided it wasn't worth going after the publisher with a lawyer.
I really need to read the whole book and check if anything else has been copied - but it costs £100!
I wouldn't do that. I would contact the publisher, identify the material you have found and tell them you need compensation for this material plus any other they have used. When they settle (royalty or lump sum) it will probably be a flat fee anyway, not per word. You would then request a copy for review before agreeing to anything.
Having been on the other end, publishing packaged software, my company made 100% sure that all rights in everything we used were cleared. Just one problem can get all your inventory seized (or a stop selling order - can't remember the legal term on that) and put you in a position where you lose the whole value of your inventory.
If you are in the USA, you have a very strong negotiating position here if you contact the publisher. If you can prove you own the material, they will be forced to settle with you or stop selling. The penalties for copyright infringement are very stiff, even if it is unintentional.