My wife and I each made our own D&D Beyond accounts, but dont want to have to buy the books twice. Is there a way to share the books with her? Like, a family account or something? I know with subscriptions you can share content in a game setting, but we want to do character building outside of making a campaign.

You can use a master tier sub to content share in 3 (or 5?) campaigns. You can have one just for sharing. Make characters in the campaign then leave it to add to other campaigns. For example if you make a character and add a shared subclass, then leave the campaign, you (or your wife) can continue to progress that character outside the campaign. They'd only need to add the character back into the campaign if they need to add something new that was shared, like a feat or item. 

This is the only way to content share.