I’ve never done this before, so I don’t know the board rules:
Am I allowed to link to a novella that a friend of mine wrote and had published for Amazon Kindle? The proposed thread would include my honest opinion of the book (which, in the interest of full disclosure, I only gave four out of five stars to).
Linking to, especially with a review, would be fine. And I don’t see the necessity of disclosing your relationship, but that would probably be a good idea. It would probably help if it was a topic of interest, of course, of the kind that is generally found around here.
We have Dopers who talk about books they themselves wrote. I even bought some of them. (And they were good.)
Still, you have to be careful not to cross the line into advertising, unless you are doing it in the Marketplace.
The difference between advertising and “a plug” is an exercise left to the reader, but graded by the moderators.