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I think discussions online period are a bunch of garbage.

Still, have you considered Goodreads?

EDIT: This isn't Show HN, people really look for that tag. Edit your title and delete it.

I'm afraid discussions are perishable things.

For instance, perhaps you read a book several years ago, that I only recently picked up. Catching people in the moment, just when they're interested in that very topic has a limited time-window. Not unlike HN threads.

you could also start writing a blog. oreilly for example support bloggers with a book review program, and usually you can find blogging peers giving you feedback. in any case, feedback would be helpful for authors too, and for my book, i thought gathering discussions on books with similar subjects on a site: http://backbonebooks.info

Idea: Online book discussions