『spoon.』の表紙絵を担当しているのも堀口さんですよ。

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Source: https://ameblo.jp/cola-tea/entry-11065393625.html
EDIT: actually that source is incomplete, Yamada contributed to the drawing as well: https://i.imgur.com/Yr1YqCI.jpg
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ps. though yamada is not credited in the artbook, but it looks like her composition
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