Translate “book number”

Hi, hope somebody reads this and can help me.

Im trying to translate the word “book” or “books” that appears before the book number in a series. I used Loco Translate to change the words that i needed, but couldn’t find the string for this line.

Also, please don’t let this plugin die, is probably the only one in it’s kind and that does what i need. It’s sad to see no communication from support here.

Thanks!