Determining wood moisture content is not like measuring the length of a nail—it is not a truly static trait of any piece of lumber, or even of a finished wood product.
The changes might be small enough to go unnoticed—your bookshelf seems pretty stable, right? Or, they might be large enough to cause some real problems: Stuck doors, buckled floor boards and split wood stock all happen because wood is capable of change—and it all has to do with moisture.