Books were stacked three deep on the mantelpiece, ...

（マントルピースには本が3段に積み重ねられていた。）
　『Harry Potter and the Chamber of Secrets』（ハリーポッターと秘密の部屋）より。初めてウィーズリー家を訪れたハリーが部屋の中を描写するシーンで。

• ここで使われている deep は、副詞。"two deep" や "three deep" という形で使い、「2/3列（段／層）に〜する」という意味になる。（if things or people are two deep, three deep etc, there are two, three etc rows or layers of things or people／if people or things are two deep, three deep etc, there are two, three etc rows of them behind or on top of one another／In a specified number of ranks one behind another）
  • People were standing four deep at the bar.
  • They parked the cars three deep [=three cars in a row], and our car was stuck in the middle.
  • People were standing three deep at the side of the road to watch the procession.　※procession：行列、行進
  • They were standing three-deep at the bar.

（左がUS版、右がUK版。私的には電子書籍版がおすすめ。）