UKH

What outer layers do you go for mountain biking on cooler summer days/this time of year? Soft-shell? Lightweight waterproof? Windproof? Any recommendations?

A very light windproof jacket or smock is ideal for this, because it will probably spend most of its time in your pack or stuffed in a pocket. Something like the Montane Featherlite Trail Jacket.

Light windproof or additional long sleeve top is all I wear at this time of year. 

Carnac vest, long sleeve Morvello MTB top, Endrura waterproof. Pack a light fleece in my bag if really out there.