Book value is purchase price less depreciation of an asset. In the case of a company, it is assets less liabilities. We report our data provider's common equity line (not shareholder equity) as book value.
The typical calculation of book value for a company is total assets less total liabilities, which results in shareholder's equity. There are several lines on the right side of the balance sheet, though, that are potentially distortions. First, some companies have a value that is not asset, liability or equity: minority interest. Furthermore, some activities result in "equity" lines that are not value-additive: Preferred shares, in particular, function much as debt, yet are included in the equity section. (And minority interest has the potential to also appear in the equity section.)
Rather than second guess all the right-side lines that could affect book value, we just draw on common sense and report common equity rather than shareholder's equity.
A side note: Some definitions of book value deduct intangible assets. We have a separate function, tangible book value, that makes that adjustment.