It's probably just not in shops because most items in shops are there because players have sold them, and mass hiding isn't there because nobody really hunts in the areas where it drops, so you'll just need to find it yourself!
Typically books will drop in areas with mobs at or above the level, so with mass hiding being a level 21 book, your best bet is to look for mobs that are level 21-30, off the top of my head I'd say omas in woo yon woods might be a good place to try