"Book Value" is more of a "At Best" value, typically reflecting auction price where people will bid up each other to claim the prize.
Realised price is sometimes higher or equal to possible value, but it's usually lower.
For re-evaluating current value, not using perpetually incorrect book-value:
The best way to find a general American value of such a collectible is, look at e-Bay's Sold Listings, which shows a realised price in green (and unsold in black if you've clicked Completed Listings to see what they're not selling for). Then add to the sold, green price the cost of shipping if it stayed in the same country (America, specifically the lower 48 states). If it went out of or is coming from outside of the continental U.S.A., the sold listing cannot be used for it has its own separate variables (everyone remember the Sheikh, who would buy bottles from several countries at very high prices?). Influencing variables to look for in the continental U.S. sold auction for determining value: Exact same item (shade of colour, applied v. tooled top, same size, etc.), in exact same condition (staining from being dug? Dirty? Cracked or chipped, a gouge, etc.), coming from a seller with at least 95% positive feedback with over 10 sales (a seller with too few sales, or not as good feedback, will not get as many people bidding as they do not trust his reputation or lack thereof).
So, to re-evaluate current value using an everyday auction site (e-Bay), there is what we'd need to look at.
But, e-Bay is not the same as a bottle auction-house. Auction-houses see higher realised prices, because people seem to believe they're getting better merchandise? Also, the values on e-Bay often deviate from those in fleas and antique-stores, albeit they often end up with unsold merchandise, or lose it to thieves/damage. But that is a whole different animal.
Lengthy, and a bit off-topic, but I think it gives a good overview of finding a value through recent sold listings (I consider 'current value' to be monthly, rather than by year. You cannot trust the value of an item 5 years ago, as being the value today. Examples of things that continue to decline dramatically in value: Porcelain and furniture).