Peter Hirtle and I have tried to analyze this over a couple of years and may write an article some day - so readers, feel free to weigh in. Meanwhile, Peter tells me that LJ announced that a million book covers are now available for download and display in library OPACs via LibraryThing. I expect libraries will be delighted to try this service.
But who actually owns the copyright to the book covers? Likely the book publisher, though it could be an artist who designed the cover. The question then is whether or not there is an exception in copyright law that allows libraries and others to scan and post images of the covers.
LibraryThing states: "Publishers and authors want libraries and bookstores to show their covers. Under U.S. law showing covers to show off books for sale, rental or commentary falls under Fair Use in most circumstances. (We are not lawyers and make no warrant that your use will be legal.)"
Minow take: I'm not aware of a court case that supports this statement, but readers please add comments if you are. It seems that Fair Use would need to be evaluated on a case by case basis. Questions should be asked such as: what is the library's purpose in posting the scan? OPAC? Reading program? Posters? How creative is the cover? Did the cover have its own copyright or is it a small part of a much larger copyrighted work (i.e. the book)?
However, there's another copyright exception that could be useful here -- the "useful article" provision at 17 USC 113(c) which states:
In the case of a work lawfully reproduced in useful articles that have been offered for sale or other distribution to the public, copyright does not include any right to prevent the making, distribution, or display of pictures or photographs of such articles in connection with advertisements or commentaries related to the distribution or display of such articles, or in connection with news reports.
and 17 USC 101 defines "useful article" as:
A “useful article” is an article having an intrinsic utilitarian function that is not merely to portray the appearance of the article or to convey information. An article that is normally a part of a useful article is considered a “useful article”
Assuming the books are "useful articles" it seems that Sect. 113 is more helpful than Fair Use. It seems that a strong argument can be made that with today's enhanced online catalogs that include book reviews, the commentary criteria is met. For items that do not have reviews attached, there is still a possible argument that the pictures are used to help advertise the book.
This assumes that "advertise" can be used in a broad, nonprofit sense - to promote checkouts of the book, rather than sales.