Multi-volume work with unnumbered volume in biblatex

Question

In biblatex, the field volumes can be used to indicate the number of volumes in a multi-volume work. But how can I indicate that a multi-volume work has e.g. 8 numbered volumes plus a supplementary unnumbered volume, e.g. an index?

Ideally, I would like to output a reference with a format like "8 vols, 1 index", "8 vols, 1 appendix " or "8 vols, 1 supplementary vol."

Answer 1

You could redefine the format of volumes to print a literal unless its a numeral. Something like this:

\documentclass{article}
\usepackage{biblatex}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@mvbook{mvbook1,
  author = {AuthorA},
  title = {Title 1},
  volumes = {8 vols\adddot, 1 supplementary vol\adddot},
  location = {Location},
  publisher = {Publisher},
  date = {2017}
}
@mvbook{mvbook2,
  author = {AuthorB},
  title = {Title 2},
  volumes = {7},
  location = {Location},
  publisher = {Publisher},
  date = {2017}
}
\end{filecontents}
\addbibresource{\jobname.bib}
\pagestyle{empty}
\DeclareFieldFormat{volumes}{%
  \ifnumeral{#1}
    {#1 \bibstring{volumes}}
    {#1}}
\begin{document}
\nocite{*}
\printbibliography
\end{document}