As the title states, my DM has shared a book with our group but I can't access a subrace within that book when I go to create a new character. So how does content sharing work in regards to content within the sourcebooks? Does he need to share that particular book in the campaign or am I supposed to be able to use the content once it is shared?

Two possibilities I see:

1. He only bought the Compendium version which has the text but not the creation tools.

2. You are creating a character outside the campaign.  The DM needs to send you an invite and you create the character with that link.

I think it's option 2. Does he have to resend the link in order for me to create a new character in the campaign we're already in? Thanks for the help btw.

If you're already in the campaign, and can see the share link on the campaign page, you can keep using that same link.

Thanks for the tip. I found the link and I'll ask the DM if it's cool if I add my character. Thanks for all the help.