REALISTIC!!!! For fiction books- something that cannot be considered “trite, already done…” Also, please stop writing books that the entire plot can be unraveled in the first few pages. I will not mention the author or title, obviously, but i read the book to check my solution and I was right on all accounts! You can put all novels in a few categories- make them more real, different, and plausible.
Non-Fiction- Realistic! Even books about great people, tzaddikim ect did make mistakes in their past and they weren’t perfect from the minute they were born and didnt necessarily know by the age of one all of chumash, two nach, three mishna and four shas. Many times I just do not read biographies because I get annoyed by them.