If a book cover has been scanned and slightly altered, it will clearly be an infringement. For example, a book cover that placed an image between the words of a title and shaved off the bottom of the type in the top word and the top of the type in the bottom word would presumably not infringe the cover of Looking Closer 2. However, we might feel on ethical and aesthetic grounds that taking part of the creative concept was nonetheless undesirable. Scanning has certainly made appropriation so easy that educators should be rigorous in warning their students. The schools are the best place to create and deepen awareness of professional standards, although professional groups like the American Institute of Graphic Arts and the Graphic Artists Guild also play important roles.
Some instructors give their students misinformation about legalities.
This isn't true, since a part of a work can certainly be infringing if an ordinary observer would find it to have been copied. In this context, it's also worth mentioning that using images without permission in the course of creating in house dummies also raises ethical questions, particularly if there is no intention of giving an assignment to the creator of the image. If knocking off a particular style goes beyond what we might call influence, as I think it does, I would say that the unauthorized use of art for in-house dummies is plagiarism and unethical. Any designer would be outraged if his or her portfolio were used by another design firm to obtain a project. Also, the unauthorized scanning of an image for use in a comp is an infringement of copyright.