Here as soft question to know what think people. I was thinking about odd perfect numbers, see for example some online encyclopedia about this subject, when I have thought that (at least I think so) there isn't in the literature a specific book dedicated to odd perfect numbers.
This is the situation, I know articles and even that are in the literature thesis about odd perfect numbers, but I don't know a book published and entirely dedicated to odd perfect numbers. And there are tons of pages dedicated to odd perfect numbers but I believe that nobody wanted to publish a book about something that perhaps does not exist.
Then I thought if there are in the literature some books or monographic entirely dedicated to some mathematical object which existence is an open problem, that is an unsolved problem.
Question. Just as curiosity, do you know a mathematical book dedicated entirely to a mathematical object (from a branch/field of mathematics) whose existence is an open problem currently? Many thanks.