Just a basic question about how Sharing works.

When someone shares books with me, it seems as though I can read them but I can't actually use them in the character builder. Is this correct or am I doing something wrong?

You can use them in the character builder, but only on characters that are actually in the campaign where the books are being shared.

Join your character to the campaign first, then go back and edit it and you'll see the appropriate options.

^ this, or you use the campaign join link and choose "Create a Character" to start with a fresh one and all the options.