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?
I take it to mean that there were three stacks of books (each stack being of indeterminate height), one behind another.
To give some cheap ASCII art of it:
||
wall
|| [[[[ ]]]] :::::
|| AAAAA BBBBBB CCCCCC
|| @@@@@ ##### $$$$$$$$ Harry stands here,
|| 1111 2222222 3333333 <- looking that way
|| abooka bbbookb ccbookc
===============================mantel==
stack 1 stack2 stack3
In the above "art", the books are stacked five high and three deep (from Harry's perspective). There's probably room for another stack, which would then make the stacks four deep; and you could pile on as many more books as you want (until they fall over), making the stacks 15 high or more.
Yes, it means the books are stacked in three layers.
The expression "three deep" (four deep etc.) also carries the sense of irregularity. The books are not precisely arranged in three layers. To take Damkerning's comment, three deep at a bar implies a throng of people not an orderly queue.
