A number of skill books are not available in the random selection from vendors no matter how many times you make them reroll their inventory - Walk in Shadows is one of them. (Dust Devil is another).

If you want one of those skills, you have to select it at start.

There actually is a copy of Walk in Shadows available as part of the fixed inventory of a vendor that may or may not become available to you, but that's pretty far into the game. In case you don't mind being spoiled:

It is on one of four elemental vendors you can unlock at the homestead, each of which has three fixed spells plus a random selection. Specifically, it is on the air elemental.

Fire: Meteor Strike(19), Immolation(13), Man-at-arms: Phoenix Dive(8)
Ice: Hail Attack(19), Mass Slow(16), Marksmanship: Arrow Spray(13)
Air: Storm(19), Make Invisible(16), Scoundrel: Walk in Shadows(2)
Earth: Earthquake(19), Petrifying Touch(8), Witchcraft: Death Punch(16)

When I said death before dishonour, I meant it alphabetically.