documentclass{memoir}
usepackage[authordate,backend=biber]{biblatex-chicago}

addbibresource{jobname.bib}
usepackage{filecontents}

begin{filecontents*}{jobname.bib}

@book { snellgrove1959,
author = {David Snellgrove},
year = {1959},
title = {The Hevajra Tantra: A Critical Study, Part 2},
subtitle = {Sanskrit and Tibetan Texts},
location = {Oxford},
publisher = {Oxford University Press},
}

@inbook { snellgrove1960, %Changed it to 1960 for the sake of the order.
author = {David Snellgrove},
year = {1960},
booktitle = {The Hevajra Tantra},
booksubtitle = {A Critical Study},
volume = {2},
%subtitle = {Sanskrit and Tibetan Texts}, %subtitle is not printed if there's no title
title = {Sanskrit and Tibetan Texts},
location = {Oxford},
publisher = {Oxford University Press},
}

end{filecontents*}

begin{document}
nocite{*}
printbibliography
end{document}