I don't think their will be sheet music either. I think you should sell a certain amount of records to have a book done. I don't know how much Opeth has sold, but I know that Satyricon's "Nemesis Divina" has sold 50.000 copies and a book hasn't been done for that. I've heard that Cradle of Filth sold 100.000 copies of one of their albums, and I don't think a book has been done for that either.
As far as I know Type O Negative's "Bloody kisses" tab book didn't come out until the record had sold 500.000 copies, which was in 1995 I think, so there's a long way to go if Opeth wants a tab book put out. Sell out and get a tab book or stay true and find tabs on the internet.