The Book Cover Archive is a thorough and expansive collection of book covers. You can browse the archive by designer, illustrator and publication. It even tells you what typefaces are used on the covers - very useful. Here you can find all of the Great Ideas/Loves series by David Pearson as well at the 70 commemerative covers by Penguin. There is a broad selection of both illustrative and typographic book cover design - all have been carefully selected and represent the best there is to offer! It's well worth a look. Here are just a few that I found - I particularly liked a series of Nabocov books published by Vintage.
'Unhealthy book fetishism from a reader, collector, and amateur historian of forgotten literature. Recent obsessions: illustration and graphic design from around the world'
It contains a great collection of narrative/sequential illustration, book design, book covers etc from all sorts of sources. It would be particularly useful for the first years who are looking at sequential illustration for their 'Entrance and Exit' project!
Like this stuff from Alexandra Grinevsky's 1929 illustrations for Valery Larbaud's "Deux Artistes Lyriques."
Lot of other stuff as well so well worth a trawl round.