...even if it’s likely to be a one-off, it can a good idea (and potentially easier to sell to a client or publisher) to treat the cover design like the first in a series, because you never know where it might lead.

A few cover collections I like (some more obviously pre-planned than others).

By Clarence P. Hornung for The Decline and Fall of the Roman Empire, volumes I to VII.

By YES Studio, London, for Penguin.

By New-York based Cardon Webb for Oliver Sacks.

By Angus Hyland of Pentagram.

And The Book Cover Archive is a great resource "for the appreciation and categorization of excellence in book cover design," edited and maintained by Ben Pieratt of General Projects and Eric Jacobsen of Whisky Van Gogh Go. More than 1,300 covers, searchable by designer, title, author, publisher, publication date, art director, photographer, illustrator, typeface, and genre.