Under the rule in the English Code (Joint-Code) this would be given as

Primrose, Archibald P., Earl of Rosebery. Napoleon:
the last phase. 1904944.05

For the former entry no reference is needed from Primrose to Rosebery, but in the latter the reference from Primrose is absolutely indispensable.

The indexer of a lately-published book on dress gets over the difficulty in a very easy if unsophisticated way by entering under "Lord," after this manner

Lord Beaconsfield, how dressed, 235
" Brougham, his check trousers, 104

It is of importance to have the books entered under the highest title attained at the time the catalogue is published. The two undermentioned books illustrate this point. The first is described as by "the late Alexander, Earl of Crawford and Balcarres, Lord Lindsay, Etc.," and the second as by "Lord Balcarres." We distinguish between them and enter in this way

Crawford and Balcarres, Alexander, 25th Earl
of. The Earldom of Mar in sunshine and
shade during 500 years. 2 v. Edin., 1882923.2
Crawford and Balcarres, David, 27th Earl of.
The evolution of Italian sculpture, illus. 4o
1909784

This latter work being published while the author was Lord Balcarres necessitates the reference

Balcarres, Lord. See Crawford and Balcarres,
Earl of.

which will fit any Earl of Crawford who might have published a book while using the courtesy title of Lord Balcarres. It may be admitted that entry by the family name would bring all books together by members of this particular family, irrespective of the titles under which they may have written, whether Lord Balcarres, Lord Lindsay, or Earl of Crawford, but such cases are few.