"Too busy" is the typical response that usually means, "I don't see the value because you didn't move me by articulating how talking to you connects with something that is highly important to me." Go back and check your conversation with the person who referred and every step in the process:
1. Did you get truly meaningful, important, significant, and / or compelling information about the person to whom you are being referred and introduced from your client?
2. Was the note on the book compelling?
3. How well put together were the talking points that you provided your client to prepare them for making an effective introduction?
4. How soon after your client meeting and referral conversation was this nicely "packaged" email forwarded to your client with these talking points?
5. What was the conversation like with your client when they called to tell you to send the book to their referrals? In other words, did you get something confirming that their referrals are expecting your call and hopefully looking forward to it?
6. When you made your follow-up phone call, how effectively did you communicate that the purpose of the call is related to what matters to them?
If all that was actually done effectively it is unlikely that you will ever hear "I'm too busy."