The latest is the Rider from Skinnycalf Boots, a just-launched NYC-based line aiming to bring "young and current but also timeless" designs to women with smaller than average calf circumferences. These Italian-made beauties were kindly loaned to me for review, and I'm happy to report they are among the slenderest riding boots I've tried.
I would say these run just on the snug side of true-to-size, so if you are between half sizes, go with the larger of the two, especially if you plan to wear with a thicker sock -- though the leather feels like it will stretch nicely to conform to the foot.
Also in the initial rollout was/is a cozy, shearling-lined wedge boot for the chilly months.
Rating for the Rider:
Calf slimness: *****/5
In cognac & black.
Find it at:
Skinnycalf.com (20% off as of October '15, with code SKINNY20)