Commentary Par XII 122

The word volume (volume), occurring first in [Inf I 84] and last in [Par XXXIII 86], literally runs from one end of the poem to the other.  It occurs nine times, and always else either refers to God's book (the Scriptures) or to his 'other book,' the created universe (except in its first use, where it refers to the Aeneid [see C.Inf.I.84]).  Thus, to refer to the slender booklet, the Rule of the Franciscan Order, as a volume is to employ a heavy word.