I think that books signed by staff aren't exactly that worthfull, or at least very hard to price. Sure, the promo ones are worth alot, but 'normal' books like this one really depend on what staff it is and how easily you can obtain them. Some mods you can just ask for their signature so then it would be the price of a book and quill .
Well---A lot of people do collect signatures, so I can see a lot of people buying that. I don't know how much it's worth, I would buy it, but I'm not willing to pay a million rupees for a book. AncientTower's signature, he hasn't made a system to sign books so he won't sign, which means that this is uncommon, BUT---He literally like JUST a week ago started selling signed books at his residence for 5k. So while this individual book is uncommon, his signature is still possible to get. This book is still valuable though, I think it's cool.
True enough I have other books but there is only one copy of this book in circulation. Here is the link to the origin of this book. There were only 10 copies but only one was found before the waste reset. https://empireminecraft.com/threads/sweepstakes-towers-100k-giveaway.72210/
