Which is worse?
Finding a book you love and then hating everything else you try by that author, or
Reading a completely disappointing book by an author that you love?
This is really hard to answer! I don't think I can actually think of anything that would fit into either category. I'm actually not often disappointed by a book, and its very difficult to say I hated one! Perhaps I'm just not a very discerning reader!
The only example I could think of would be Ian McEwan. The first I ever read was Enduring Love, and I loved it, and nothing else that I've read by him has ever matched up. But I've by no means read everything he's written and I didn't hate any of them. I just didn't love them like I loved Enduring Love.
In terms of hating a book by an author I love, its never happened! I think all authors have a weakest work, or just one that doesn't appeal personally. Taking the question on this level, I would have to say Margeret Atwood's Surfacing disappointed me a bit. Everything else I've read I've loved, I just wasn't as enamoured with this as with the others. But again, I've not read everything she's written yet, although I'm coming close.
I think that is a bit of a cop out answer really, but it would take a lot for me to say I hated a book, especially if I had an affection for the author.