I found a public domain book that has never been re-published and I wanted to include illustrations with the book. If I submit manuscripts to publishers, how can I protect my idea? What if they just use their own illustrator and take the book themselves from public domain? Is there anything I can copywrite? Or trademark? Such as, the authors name, the book title, or characters?
I hope this makes sense and appreciate all the help with this.
No. There is no protection for literary ideas, only specific recorded works of literature. If the work is in the public domain it is in the public domain and your idea to publish it with illustrations is not something you can protect or prevent other people from doing. Multiple publishers publish well known public domain works in various guises and there is nothing any of them can do to prevent other people from doing the same.
If you edit the existing work, the copyright to the edition you create belongs to you, but that does not prevent anyone from creating and publishing their own editions. If you create the illustrations, then the copyright on the illustrations belongs to you and no one else can publish them, but you can't prevent anyone else from doing their own illustrations. In short, you can protect actual work (it is protected automatically) but you cannot protect an idea.
(It should be fairly clear why protecting ideas is not practical. If it were allowed, people would scribble down hundred or thousands of idea and then just wait for someone to produce an actual piece of work that was close to anyone of those ideas, and then sue them. It would be like patent trolling, but without the pesky business of actually having to invent something. Therefore only actual work is protected.)
On the other hand, publishers do not rip off people who bring them publishable ideas. If you bring them a good idea that makes them money, they will want to work with you again, so you can bring them another idea that makes them more money. People do sometimes feel like they have been ripped off by publishers because sometimes works are published that are more or less like an idea that they pitched to the publisher previously. But the fact of the matter is that the same idea often occurs to multiple people at once, often sparked by some current event or pop culture trend. This leads to may false claims and many groundless fears. But the fact is that publishers love it when people bring them publishable ideas (most of the ideas they receive are hopeless non-starters and sifting through them is expensive and time consuming).
All this said, IANAL, and if you want professional advice, consult an intellectual property lawyer.
