After failing miserably at his "terrorism expert" job, Clark is pushed out of the loop by the Bush admin. He then "retires", joins the Kerry camp, and writes a book.

Obviously, he can say anything he wants in the book about "private conversations" and lie about other things as well. The president (historically) is in no position to sue anyone.

BTW, his book is owned by Viacom who also own CBS and "60 minutes".