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