My golden rule--if a book goes to pot, even if it has your favorite character in it, drop it like a hot potato. That's the only way you are going to send the powers that be a meaningful message.
In a similar light, if you aren't getting "x" amount of enjoyment from a book that costs "x" dollars, drop it too.
Use that freed up cash to try something new. You might like it better than that original book.