Dear Rich: I am creating a computer program that requires a lot of line art. I found a series of books ('Scan This Book', 'Scan This Book Two', and, of course, 'Scan This Book Three') that display excellent public domain line art. The legalese at the beginning of the book states that all of the art is copyright free, but that the selection and layout is copyrighted. My program has nothing to do with distributing clip art, and I do not need to use the layouts used in the books. If I were to use most of the art from these books, would I be infringing on the author's/publisher's copyright? Would it matter if I slightly altered the images by coloring them? Would it matter if I combined their selection of images with other public domain images? The short answer is that you're fine copying the material in the book. Like all-you-can-eat restaurants, owners of public domain collections don't mind how much you digest, they just don't want you taking all their stuff and selling it somewhere else. Assuming your software product is not created to sell public domain art, there's no reason to bother with any of your other questions.
Copyright Rules. As you know, you can do whatever you want with public domain artwork. However, under copyright law, the owners of the book may have a compilation copyright. We say "may" because compiling public domain works doesn't automatically amount to a compilation copyright. There must be sufficient creativity in the judgment, selection and arrangement of the public domain material -- for example, "The Best American Stories from the 19th Century" or "Sketches of Victorian Kitchens" are probably protectable because they require decision-making and selections. Even if the book constitutes a copyrightable collection, that copyright is still regarded as being "thin" -- meaning that you would need to lift nearly all of the thing "as is" to infringe it. Public domain compilers such as Dover are primarily concerned with someone taking their material and competing with them. In summary, if you are using the public domain material for aesthetic or decorative purposes and not redistributing the artwork for re-use or re-sale, you're free to do whatever you want.
BTW. The Dear Rich Staff believes that the Scan This Book series is out of print (we couldn't find new copies available online and we couldn't locate the publisher Art Direction Book Company). That doesn't mean nobody will assert copyright; but it does indicate that it's not at the forefront of a publisher's radar.