MrX is also the co-author (with MrY) of A Brief History of English Underwear, published 1981.
Just one question: Isn't ''also'' redundant if we already have ''co-author'', ''co'' meaning with sb else, or that somebody else also wrote the book?
Antonia - I think the "also" in Mr.P's sentence means, "in addition to whatever book we were talking about earlier, MrX was also responsible for A Brief History of ......"
The "also" refers to Mr.X's other book, not to Underwear's other author.