How do I use custom post-types to create main/sub-posts?

I have a dream, but I don’t understand the inner-workings of WordPress enough to fulfill it. I’m hoping someone can point me in the right direction.

The dream:
I am building a niche social bookmarking site for Fan Fiction readers, where members will be able to review the stories they’ve read, read other reviews, favorite their stories, and bookmark stories to read later.

I think it would be a waste of DB space to have the same url listed over and over, which will happen if a story is popular, so I want to separate out the links from the reviews and make that part re-usable. So, there would be several types of post-types and I assume, they would be connected through hierarchy, but I don’t know how to set that up and I’ve looked through the Codex and I don’t think it’s as explanatory as it could be.

I think the post-types would be something like:

bookmark – story title w/ author name (title), url (custom field), summary (post body), tags

review – reviewer (post author), review (post body)

favorite – notes (post body), tags

to-read – notes (post body), tags

So, the idea would be that the bookmark “post” would appear once, but members/users could select it for their favorite or to-read lists or add tags to it or add their own reviews once it’s in the DB. Obviously there would have to be a function to check the DB for the url when someone enters it, but that can’t be too hard…

I am using the latest versions of WordPress, BuddyPress, Gravity Forms (though I haven’t figured that out either) and Suffusion as you can see at https://j2-spn.exit-23.net.

If anyone can explain how to make this work, I would be extremely grateful. Thank you in advance!

J