Of all the reason that one would read the book about poisonous mushrooms she chose the assassin reason which is silly cause for the simple fact that there are more deadly and undetectable poison out there. Assassins will mostly chose fast acting as slow acting has higher chances of being cured and possibility of giving damning testimonies to authorities and media.
First thing that comes to my mind is people read those to know which mushroom to avoid even when buying at supermarket as some might slip the inspection.