Lets assume you think your work is being copied, is it taking snippets of text without adding its source.. Or......
Is it copying word for word or your images.....then its a copyright infringement. You should first send a request to the author/ publisher to remove the book and at the same notify your actions if they persist. You can contact the iBooks support team and pass on the information of the "offender".
I forget where I saw it, but I am sure Apple state that the publisher must infringe copyrights, that aside I am sure it would be too difficult for Apple to check every book content for validity.
Copyright is not always as straightforward as it is thought to be, and different countries have different Copyright laws. The USA seems the most "lax" with its Fair Use policy.
The UK now offers a "cop out" in that one can contact, even by eMail to ask permission to use, if they don't reply, then they cannot later complain when dated proof of a request is produced.