Book value is not a very relevant valuation method for young biotechs. BV is really for companies with a lot of physical capital (companies with heavy inventories (eg Best Buy), real estate (mall owners), financial assets (banks) or little goodwill or other balance sheet intangibles). The idea is that BV would be a rough approximation of the liquidation value of the company should it go insolvent - the BV acts as a floor and you'd use it as a comparative to other peer companies valuations to determine if the stock is relatively undervalued.
The value of a pre-revenue young biotech, however, is embodied almost wholly in its potential future cashflows. This makes valuation of the company EXTREMELY difficult and subjective. Add in the fact that they are subject to extremely binary information events (trial results, regulatory decisions etc) and you have the recipe for high equity value volatility if their shares are publicly traded. Sound familiar? :-)
This is why a biotech investor has to be prepared for volatility and must be vigilant and patient. Not easy, and this does not fit the description of most retail investors, who really should stay away for the sake of their peace of mind. It is why the conspiracy theories start to fly. People feel compelled to "explain" why the share price is moving so much since it seems there is no reason for it.
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