I have had a few books shared with me from a campaign and was going to create a new character.  When it comes to picking a subclass my options are limited (ie as a rogue i can only pick thief) but i assumed I’d have all the options from thh ePHB if it had been shared with me.  Was i wrong in thinking this?

Is the character you're making inside the campaign where that content is shared?

Content-sharing is campaign-specific. So you need to have the character in the content-shared campaign in order to use it.