Having such a tome would spare 18 slots. The tome contains three scrolls upon purchase. Scrolls in the inventory are not added automatically to the tome; it must be dragged manually to it in order to add it.
Using up all 20 scrolls will make the tome unusable, but new scrolls can be added to it at any time. Scrolls picked up or bought while the Tome is in the inventory are automatically added to the tome, unless the maximum number has been reached.
Right clicking on the tome will open a town portal in the same fashion as a normal scroll.