I definitely think that books should have content warning. I do think that it's only logical that books should have come kind of warning for people to know what it is they're about to read. I think that some form of arts such as books need to have a warning to prevent younger readers from reading them.
I think it is only fair that the reader knows to what extent sexual or violence is in the book, some people can't stand those subject matters and they wont be able to finish the book that they purchased, I think content warnings are a great idea that only help not hurt the book industry.
It would be great if books had content warnings much like those for television shows and movies. Many of the Kindle books I look at mention in their descriptions if the books have a lot of sexual content, extreme violence, or other graphic content that some readers may wish to avoid. The content warnings don't have to be super detailed, but authors and publishers know if their books have sections that some people might not be comfortable with. Content warnings would really be appreciated by people like me who love to read, but really don't want to get involved in a book where every other word is objectionable and there are detailed sex scenes that would make a doctor blush.
Yes, readers should be able to determine in advance whether a book contains material that they might find distasteful or objectionable. Systems already exist for self-regulation of films, video games, music, and television; it should be relatively simple for publishers of books and magazines to adopt and implement a similar system.
No, books should not have content warnings, because people can read a bit about the book and find out what the book is about. Anymore, any book can be previewed online. Other people post reviews, and you can see if there is any questionable content in the book. It is up to parents to monitor for their children.