。
A book fell from the shelf.
Literal
Book [subject-が] 1-[counter-冊] shelf [from-から] fell.
一冊 is '1 book' using the counter 冊 for bound volumes. A syntactic quirk of Japanese: counters appear detached from the noun they count — 本が一冊 ('book, one-volume'), with the quantity trailing behind the noun rather than preceding it as in English. This is called the 'floating numeral quantifier' construction.