There are a lot's of factor that could affect the price of a company without appearing in its book value: growth perspective, crisis, interest rates, regulation, price of oil, management style, branding, R&D, marketing, goodwill, etc...
For example: How much is worth a marketing effort done one year ago? Does it still reward the company? How much is it worth to have well known and respected products? How much are worth clients satisfaction and how does it affect your value?
Because of that, even if we had "perfect markets" the book value will still not be equal to the market value.
Let's take ebay: it does not own more than a bunch of computers for their websites.
What is the value of the website "ebay"? How do
you put it in the book value? Is it worth as much as the computers
that host the website (obviously, no)? Should it be the price of the R&D
effort put into the website? Without marketing no one would know
what ebay is, so the marketing effort should also be taken into
It's very hard put a price on that kind of assets.
That is why there many methods to assess the fundemental value of a company (ie, what should be its market value). The 3 most known methods are:
Net Asset Value, which is basically the book value.
That method could work well on real estate company whose value is more or less the buildings that they own minus what they borrowed to acquire them.
Comparable analysis, where you compare companies between them to asses their prices.
Example: Some car making companies are being traded at a PER of 15. Then, if you see a car company that, all else being equal, is trading at PER of 10, you could say that its mispriced.
Discounted Cash flow, which is saying that a company is worth as much as the cash flow that it will give me in the future
If you think that a company will give some cash flows for a certain period of time, then you compute the present value of the cashflows and say that it should be its value. We can note that this pricing method takes into account growth perspectives, which is not part of the book value.
There is another well known method, a more quantitative one, this is the Capital Asset Pricing Model, its about looking at how a company should be priced relatively to a benchmark of other companies based on past statistics.