The term 'gross' is also used in accounting. It means 'total', w/o deductions. Is it possible that grossing books is used in a financial context? Like top grossing books - best selling books?

Could it be that you may mean "Engrossing"? That is the only thing I can think of with the context of Grossing books. So, it would be an Engrossing Book - which means it is a book you find difficult to stop reading.

There’s many definitions, look at the example of gross as a verb but just reading “grossing books” doesn’t make sense to me.