～冊
～さつ · ～satsu · JLPT N5 · noun
book
📚 冊 = bamboo slips tied with cord, the original bound book
The kanji 冊 shows vertical writing strips strung together by a horizontal cord, which is how early books were made. So satsu counts bound volumes: 一冊 (issatsu) one book, 二冊 (nisatsu) two books.