Even with multiple edits and proofings, spelling mistakes can still hide in all books, so we are always happy to hear about any of these bugs that you find as it makes the book better for everyone when they are corrected. To make things easier and to avoid ambiguities, please make note in the email;
Subject of email: "Spelling correction for 'Bookname'"
The book (and version if applicable)
The chapter number
The whole sentence
The word that is incorrect within that sentence, and perhaps a note if required.
Many thanks for taking the time to do the above, we do sincerely appreciate it.