Books were stacked three deep on the mantelpiece, books with titles like Charm Your Own Cheese, Enchantment in Baking, and One Minute Feasts — It’s Magic! And unless Harry’s ears were deceiving him, the old radio next to the sink had just announced that coming up was “Witching Hour, with the popular singing sorceress, Celestina Warbeck.”
Does it mean the books were stacked up to three layers? Can you apply this usage in other numbers? like "two deep?