From American Thinker:
Relying on inside sources, quite possibly Michelle Obama herself, Andersen describes how Dreams came to be published -- just as I had envisioned it in my articles on the authorship of Dreams. With the deadline pressing, Michelle recommended that Barack seek advice from "his friend and Hyde Park neighbor Bill Ayers."
To flesh out his family history, Obama had taped interviews with various family members. Andersen writes, "These oral histories, along with a partial manuscript and a truckload of notes, were given to Ayers." Andersen quotes a Hyde Park neighbor, "Everyone knew they were friends and that they worked on various projects together. It was no secret. Why would it be? People liked them both."
Andersen continues, "In the end, Ayers's contribution to Barack's Dreams From My Father would be significant--so much so that the book's language, oddly specific references, literary devices, and themes would bear a jarring similarity to Ayers's own writing."
Christopher Andersen was on CNN defending this and other claims in his book.