Use Arabic numerals for the volume

Warning²Federal courts, Illinois courts21st and 22nd editions

A multivolume work numbers its volumes in Arabic digits ahead of the author crediting the whole work: "3 Ronald E. Mallen … Legal Malpractice 101 (2014)", not "III Ronald E. Mallen …".¹

1. 1B3 Subdivisions. Tables: R. 3.1(a) Volumes; T16 Subdivisions.
2. 2A warning: the form is usually wrong under the rule in note 1, and context can justify it.