Your right - book value is easy to quantify in terms of calculating a specific number. Your calculation yields book value per share. Thanks for noting my inaccuracy.
What is more difficult is relating that number to "value". In an extreme case, a company could be sitting on highly valuable real estate acquired years ago. The "book value" of the property could be quite small; where as the true value would be much greater. And calculating return on equity does not really reflect the true return on equity. Conversely, a company could be have an impaired or obsolete asset, the impairment of which is not reflected in book value.
Determining these factors is very difficult for the average investor.