Here are some usefull links for Excel Services 2013 for SharePoint 2013; If you find any other please update it!


Excel Services in SharePoint 2013


What's new in Excel Services for developers


Excel Interactive View


Consuming OData feeds from Excel Services 2013 in PowerPivot


Create and share external data connections by using Excel Services


Business intelligence capabilities in Excel Services

Excel Services Configuration for PowerPivot in SharePoint 2013