They say you should give things to the person who's busy. They'll get it done. However, in some cases, you simply can't add another todo. I was already juggling 12 balls when a fellow-consultant asked me to review her lengthy book. I wanted to, but couldn't, so this is how I replied.
To: Sharalyn Weldy
From: Bill Frank
Sent: June 1, 20—
Subject: Thank you for your offer . . .
Sharalyn,
Thank you for your gracious offer to have me review your manuscript. Since we first spoke, I've taken on a large corporate project which will preclude my participating.
Your book and concept look very good, and I wish you the best of luck in your publishing venture.
Thanks,
William S. Frank