So, on to my speculations about the object: *Spoiler alert* I think 'golden key' was a fine guess since it could open a way to Faerie but my guess 'golden thread of life' worked even better because it was how Sookie ultimately used the object - to bring someone back to life. I suspect that Sookie could have used it to bring Eric back to life, which I think is still a good hunch, in spite of Harris saying 'once a vampire, always a vampire.' Any object that can overcome death is a powerful item. However, it seems clear now that Sookie is not going to find happiness with Eric and Harris' answer reflected her intention that he would remain a vampire and did not reflect what the object could do. This outcome has made some fans of the series unhappy and probably accounts for some of the negative reviews.
I don't like that ending myself but I find the novel was still written with the usual quality of the previous eleven books.