This was the reason I sought out a character editor. I realize it's cheating, but I don't think it's overpowered to get Walk In Shadows this way since it appears the devs just overlooked putting the book in.

I used LSTools to add this skill to my rogue hireling and to re-assign some of his and Jahan's stats. There's a whole other thread for this, but the tool is located here:

https://dl.dropboxusercontent.com/u/32263228/lstools.zip

Instructions here on this thread:

http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=521295&page=4