Notable cross-references in the index of Donald Tovey’s Essays in Musical Analysis, Volume VI, 1939:

Agnostic, see Dachsund.
Appendicitis, see Cadenza.
Critics, see Experts.
Experts, see Critics.
Giraffe, see Berlioz.
Hedgehog, see Brahms.
Monster, see Loch Ness.
Noodles, see Brahms on plagiarism.
Pope, see Bruckner.
Sneeze, see Cherubini and Beethoven.
Sugar, see Grocer.
Witchery, see Mendelssohn.

Evelyn Waugh owned a translation of Tolstoy’s novel Resurrection for which someone had composed “a particularly felicitous index. The first entry is: ‘Adultery, 13, 53, 68, 70’; the last is ‘Why do people punish? 358’. Between them occurs such items as: Cannibalism, Dogs, Good breeding, Justification of one’s position, Seduction, Smoking, Spies, and Vegetarianism.”

See Memorable Indexes, More Amusing Indexes, Reference Work, and “He Suddenly Dropt Dead of Heart-Disease.”