The amount of cash flow equal to book value is an exchange of one asset for another of equal recorded value and therefore is not recognized as a gain, a revenue, or an other financing source.
Book value is when you exchange an item for something of greater value. 
mnli hypothesis: Book value is when you exchange an item for something of greater value.  premise: The amount of cash flow equal to book value is an exchange of one asset for another of equal recorded value and therefore is not recognized as a gain, a revenue, or an other financing source.