Still Lifes. As for your "still life" concept, the safest course is to get permission for the book cover art. At the same time, if someone says no, or you can't locate the owner, you can probably get by without permission. You have a strong argument that your work is transformative and constitutes fair use. BTW, the Dear Rich Staff isn't sure what you mean when you write of using "artwork by artists who illustrated the book(s)." If you have their permission for the additional art, great. Otherwise, you might be pushing the fair use boundaries by reproducing non-cover illustrations.
Trademark and titles. Single book titles are rarely protected -- that's why your brown bag approach is okay. However, Harry Potter, because it's a series, is federally registered and Warner Brothers owns the rights for calendars (Reg. No.3419797). For that reason, you might not want to use Harry Potter and the Order of the Phoenix on your cover, packaging or advertising. That also puts you in a better position to argue that your internal use of Harry Potter is editorial and non-infringing ... should the issue arise.