Well, smut manga typically features a lot of sexual imagery and scenarios. Regular manga, on the other hand, can be about anything from superheroes to school life and doesn't focus mainly on sexual themes. The main difference is the emphasis on sexuality in smut manga.
One example could be 'Love Hina'. It has a great story about a young man and his relationships with various female characters in a boarding house. There are some romantic and comical situations that border on the more 'smutty' side at times. The character development and the overall plot of the protagonist trying to find his place and love make it engaging.
The plot itself should be engaging. It can't just be a series of sexual scenes. Take 'Nana' for instance. It has a story about two girls named Nana who are on different paths in life. There are some romantic and passionate relationships in it. The overall plot about their friendship, career aspirations, and love lives holds the story together, and the 'smutty' elements are just part of the bigger picture of the characters' lives.
Yes, there are plenty. Project Gutenberg is a great place to start. It has a huge collection of classic novels that are free to access. You can find works from authors like Jane Austen, Mark Twain, etc. Another good option is ManyBooks, which offers a wide variety of free e - books in different genres.
In many places, smutty or pornographic content is not legal, especially if it violates moral and ethical standards. So, generally, such free smut short stories are likely to be illegal in a proper legal context.