This came up in a paper recently submitted and commented on by the tutor. The paper relied on, and separately referenced, multiple separately authored essays in a single volume ("edited by...") on a single subject.
A comment returned was "... perhaps too much reliance on a single source (though the defining volume) ...".
This was a surprise as I thought that the separately authored (and credited) essays would each be considered a separate source (though published together).
Is this, perhaps, field dependent?
(This question is not in any way going to lead to any complaint about the remark whatsoever. I just would like to know - as I said, I believed otherwise.)