If I recall correctly from when I worked on the "All Skillbooks Purchasable" mod, Walk In Shadows is classified as a "unique" skillbook, such that it is never in anything randomly generated (ie only one can be purchased from that specific vendor, and that's it).
I think there were around 20 skillbooks that were 'unique', not counting the Dust Devil skill which doesn't even have a Skillbook at all.
