what is "with B. Winter" here?

Without more information, i assume it means that B. Winter is a co-author. You haven't told us who "His" refers to, so i can't say that to a certainty.

It means "he", whose name you did not give, but who is Fernando Henrique Cardoso, did not write the book "The Accidental President of Brazil" alone. He wrote it with another person called B. Winter. This might mean that they wrote it together, or possibly that Winter interviewed Cardoso one or more times and then did all the writing himself.

Google is your friend.

The Accidental President of Brazil: A Memoir by Fernando Henrique Cardoso with Brian Winter.

http://www.amazon.com/Accidental-President-Brazil-Memoir-ebook/dp/B0014ENOG8

It usually ain't worth the candle to me . . .