日頃は冷静な乾さんの裡から、ちらりと激情が見えたようだった。
What is the reading and meaning of 裡 here? I found the readings うち and うら in Jisho.org
According to ふりがな文庫, the most frequent kun-reading is うち. うち is mentioned in this entry, but うら is not mentioned in this entry. However うち, うら and なか equally make sense in your sentence, and I don't think it possible to determine the reading in one way without furigana. They are fairly rare kun-readings, anyway. FWIW, I knew only the on-reading, り, as in 秘密裡.