I am struggling so much with something that should be really simple.

I have created a new campaign. I turned on content sharing. Then I removed some books. Saved it. Then returned to it, and disabled a few more books. But now these changes don't go through. When I try to create a new character the blocked books still show up as options. I have tried to reload, log out and back in, and add/remove more books, and nothing works. 

I've had this issue on multiple occasions now, where player's either can't see the content I have shared or can see content I have blocked. What am I doing wrong?

 Ps. I tested this with a freshly made account too. It couldn't see the content I shared with it.

Content sharing doesn't block anything in the builder and never has. That function is still aspirational. What it does is prevent folks in the campaign from reading the sourcebooks -- ie, the adventure modules.

Three things:

1. What content are you sharing? Sometimes special toggles need to be switched to activate some content for a character sheet.
2. Sometimes your or your players “Account Entitlements” can become unsynced which can prevent content from being shared properly. Follow this link to ensure your entitlements are synced: (https://www.dndbeyond.com/account/marketplace).
3. Sometimes a campaign just goes a li’l wonky, in those instances turning content sharing off and back on again usually fixes it.

I spent an hour yesterday fiddling with it, and it just has its own life. I tested by creating an campaign with my main account and then inviting a fresh account that have no books at all. 

When I turn sharing off, it shows the default few books, as expected. But when I turn it on. It shows books I haven't even shared in that particular campaign. It's really weird and it doesn't make sense to me. 

So I think I've given up and will have to do the restriction manually outside of DNDbeyond with my players. 

There is no way to restrict player options by book on DDB, only the ability to restrict Compendium Access.

I'm trying to figure out how this works, as well.

I own the Dragonlance sourcebooks on DNDB, but I don't want to share the text of the SotDQ adventure with players. If they join a campaign I start but I don't have any resources shared out, does that allow them to access the character builder components and the reference information about the new things?