He would not lose the rights to the characters. The characters in a series of literary works are eligible for trademark. He would just lose the right to preve

He would not lose the rights to the characters. The characters in a series of literary works are eligible for trademark. He would just lose the right to prevent others from making exact copies of his older books. Derivative works with characters that are not important to the canon works would be allowed, but main characters would have to be licensed before publication, lest a C&D letter render the entire print run unsellable. A breakdown in license negotiations means running a search-and-replace on some character names. Instead of writing about Jon Snow, you're writing about Dan Sleet, a suspiciously similar character. In reality, this only matters for fan-fiction, multi-author universes like Star Wars or D&D Dragonlance, and cross-media adaptations. Most print-published authors have zero problems creating original characters. The problem is making them follow the plan for the plot.