I am deeply attached to the ERB Mars books -- at least partly out of nostalgia; they were (after Narnia and Wrinkle in Time, and contemporaneously with Lord of the Rings)part of my first introductions to sci fi and fantasy....And all the paperback editions that I bought had cover art by Michael Whelan, whose work I also <3 <3 <3. The paperbacks are no longer in my possession, but I have the whole set in hardcover omnibus editions.