Artist buddies like Donato have been rocking the toned sketchbook for ages, and I am only now just catching on! Using soft leads (Or ball point), 4B and softer and hitting it with a Generals White Chalk pencil. (Though the Derwent Chinese White works too, only harder to work back over if need be.)
I am trying to embrace the softness more in my sketch work. I am usually so... Karate Chop in my attack. I am enjoying sneaking up on it and petting it rather than bludgeoning it.