Hello,
Thank you for your question.
You can do that from backend.
Go to admin area - BookLibrary : Language Manager - put Publisher in Value constant
or _BOOKLIBRARY_LABEL_MANUFACTURER in constant,
select language and rename as you need.
More details How to use Language Manager in Book Library Joomla
ordasoft.com/how-to-use-language-manager-book-library-joomla
Thanks