"*number*... and sometimes books are given numbers in a named series"
"*volume* The volume of a journal or multivolume book."
from the original manual (Patashnik, 1988, pp. 10,11)
ftp://ftp.mpi-sb.mpg.de/pub/tex/mirror/ftp.dante.de/pub/tex/biblio/bibtex/base/btxdoc.pdf

I think it even effects the display of the bibliography in LaTeX/BibTeX: volume for a book will be shown and number as well as series name for book not (in standard style).