Passport Book Number is normally called the inventory control number. Now please do not bang your head if you do not find one on your passport because it’s fine, every passport necessarily does not have this.
Depending upon the country that issued your passport Passport Book Number on your passport may vary. In situations where you have to know or write somewhere the Passport Book Number for some visa issues, Please contact the issuing authority if you are unable to come to a consensus whether or not your passport contains a Passport Book Number. Most of the situations where you have to write one will say “do not apply” so use it if you do not have one but do not panic.
Because I am an Indian, I can vouch and say it does not apply for Indian Citizens and as far as other countries are concerned, please check with the issuing authority.