Books take a long time to read. It's no surprise you feel invested in them by the time you reach the end.
Sometimes I think it's healthy to approach most books with the attitude, this is probably bullshit. I mean, it might not be the most generous attitude, but in an age of such abundance, we have to be a little selective, don't we? And then if an author is able to get past this filter, it's more likely they really do have something meaningful to say.
hmm, I find myself more applying that filter when I'm choosing whether or not to read a book in the first place, and then It's usually based on a recommendation from a trusted source or from hearing the authour make a compelling case/summary of what they are arguing. And of course, the longer the book the more actual investment we feel in them! Just another psychological mechanism to be vigilantly mindful of.