I'm sure a lot of you have opinions about copyrights. I do too. But in order to get my dream together, I had to put some effort into understanding the way they work right now.
In general, copyrights determine who has the right to profit from a work. Typically, this is the author/artist/creator, but "work-for-hire" and similar arrangements can result in the copyright being owned by someone else. (All the images used on my Zazzle products were created under just such an arrangement).
And profit is a broad term in this case. This can be the profit derived from getting the book printed and sold, either by a publisher or (increasingly these days) yourself. A creator also has the right to profit from derivative works (sequels, adaptations, etc) based on their creation.
There are ways that someone who isn't the copyright holder can profit from a copyrighted product, called "fair use" but they are fairly limited. It is fair use to provide portions of the copyrighted work in reviews, as long as you don't give too much away. A parody or satire of a work can functionally replicate it in its entirety, as long as it's for comedy. Transformative works (those that draw from several sources in order to create a new whole, such as a collage or "found art", as well as those mashups on YouTube where Buffy the Vampire Slayer and/or Blade fight Edward Cullen) may also draw freely from copyrighted works.
("Fan works", such as fan fiction, are something of a grey area. Technically, they are derivative works, but both fans and creators are studiously avoiding putting that to the test. Fans are carefully not profiting from their works, while creators have internalized what the RIAA has taught everyone by now: It's bad PR to sue your fans. I know of at least one effort to get fan work recognized as "fair use" under law, but I haven't seen any direct results of that.)
Once the copyright has expired, however, all bets are off. Anyone who wants to may copy the work to their hearts content and sell to anyone who will buy. Adaptations and sequels no longer need approval or payment to the creator before they can be produced.
The tricky part for an Oz fan is that some works have fallen into the public domain, while some remain copyrighted. Most notably, the original novel, The Wonderful Wizard of Oz, is in the public domain, while the film adaptation, MGM's The Wizard of Oz, is copyright protected. So any derivative work needs to stay very close to the novel rather than the movie to pass copyright muster.
Most notably, Dorothy's shoes in her first story are Silver, not Ruby. And most depictions try to skew Dorothy to be rather young, rather than resembling the teenaged Judy Garland.
The infuriating thing about this whole Zazzle mess is that I did all that research to make sure my depictions and presentation were legal, only to be shot down by someone who likely didn't.