Compared with the Camino Francés there is relatively little information about the Vía de la Plata available online. That said, things have improved greatly in recent years.
Perhaps the most useful resourse is the lively Vía forum at the Camino Forum website. Although caution is advised, this forum is mainly dedicated to the Camino Francés and sometimes people reply to Vía queries based on their experience on the Camino Francés thinking it's basically the same - it isn't.
Come and visit us at our Facebook page.
The Amigos de la Vía de la Plata in Seville provide a multitude of services for pilgrims, including publishing their own guide book and selling credenciales. Their website is here.
Gronze is another excellent resource for information about the Vía.