Interesting, I've thought (as most probably have) that one of bitcoin's functions is as a 'tech stock', but never that it is purely a stock. I think it ends up being a matter of semantics, yours is a valid description and is true, but things are named according to their use. E.g. A wooden table is just a flat piece of wood, it could be referred to as 'firewood', which it is, but we call it based on what it's used for. So ultimately it depends on what bitcoin ends up being used for. Right now its used as a currency and a stock, and I'll propose that it will be for most of it's existence. But it's use as a currency is using it as a stock, as you say, and I agree, so both 'currency' and 'stock' are true, but more accuracy can be obtained by using both. I use it as a tech stock and occasionally as a currency. That just makes more sense than saying "I bought a book with some bitcion shares"... because you don't buy books with shares.
I too am unaware of any bookstores that accept the transfer of company stock towards the purchase of a book; that said, wikipedia accepts the transfers of stocks for dollar value donations, so using stocks as currency does happen, it's just more impractical with all the legal red tape. That will change with systems such as Ripple and OpenTransactions, where the "contracts" and "laws" are all based on mathematics, encryption and code itself.
