Overview:
https://azure.microsoft.com/en-us/services/expressroute/
Technical Documentation:
https://azure.microsoft.com/en-us/documentation/articles/expressroute-introduction/
FAQs:
https://azure.microsoft.com/en-us/documentation/articles/expressroute-faqs/
Link a virtual network to an ExpressRoute circuit:
https://azure.microsoft.com/en-us/documentation/articles/expressroute-howto-linkvnet-classic/
Linking multiple subscriptions to one ExpressRoute:
https://azure.microsoft.com/en-us/blog/enable-multiple-subscription-expressroute/
Router Configuration Guide:
https://docs.microsoft.com/en-us/azure/expressroute/expressroute-config-samples-routing