The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 12, 'Preparation of Indexes'
12.2 What to Index
Most scholarly indexes should include subject-matter as well as names. It is much easier to compile a name index, but the reader of a book on America in the 1960s who needs to know about mixed marriages or monetary policy, and who finds nothing in the index between ‘Miller, Arthur’ and ‘Monroe, Marilyn’, will feel cheated, and with good reason.
Names of authors and critics whose work is engaged with (quoted, disputed, mentioned in the main text, etc.) should always be indexed. But if a note simply cites a source, perhaps to justify a remark made in the text (‘Baudelaire had a difficult family,10’), then the author of that source would not normally go into the index. Similarly, do not index bibliography entries. Index literary works under their authors (e.g.: ‘Eliot, T. S., The Sacred Wood’) unless they are anonymous, or (as in the case of some medieval texts) much better known under their titles.