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: