I am thinking a lot about walking at the moment in preparation for an article I am writing for TGO magazine.
There are several journal articles I've been reading but particularly useful has been:
Dynamic Principles of Gait and Their Clinical Implications
which is a review of the biomechanics of walking.
Most of my thought is shaped by the principles of POSE though and Romanov's key fundamental that we are mainly trying to deal with and work under gravity. His piece on Pose walking technique has been useful and is similar to the ideas of MovNat.
Mark Sisson's piece is valuable too.
It all builds together - a more efficient gait will allow you to go further with less effort and with less potential for injury.
I FORGOT - This ebook is phenomenal
Here are a couple of videos on technique too: