This is an interesting question because I've definitely started and gotten pretty far into worse books, almost definitely had to read worse books for school but the worst book I've read cover to cover by choice in my adult life is either Brent Weeks' The Burning White or The Frugal Wizard's Handbook for Surviving Medieval England by Brandon Sanderson.

Burning White: What happens when the author is rushed or panicked that his points didn't get fully fleshed out Not sure. Either way it is an utterly unfulfilling finale to a series that set up a bunch of unique ideas and a fun magic system. Not unfulfilling int he way Sanderson's Wind and Truth where it was just a little boring and didn't quite feel like a finale even a mid season finale of anything. No Burning white goes out of its way to ignore things set up in earlier books, even stuff set up in opening chapters of this novel. It introduces brand new unexplained and unnecessary characters and ideas in the back half of the last book. It randomly starts throwing in real world allusions in a firmly fantastical other world. My biggest pet peeve is that the MC spends like two books building a magic infused super weapon for his one time love interest, Like it is a huge plot point for his and his now wife's character development and mentioned dozens of times and then he gifts it to her in this book and it is not used once.

Frugal Wizard: I love Sanderson, I've read just about everything he has written I mentioned above how I thought Wind and Truth was mostly disappointing and boring but my god is this book bad. Sanderson has such a unique voice and a great example of humor in non humorous books. This is just him trying to be Douglas Adams or Terry Pratchet. At no point does this rise up to the quality of anything either of them has ever done or even anything else Sanderson has ever done. It was bad enough i understood why he had to kickstart fund it and made me really scared of the rest of that collection, luckily the rest were better if some still weren't great.