Death spells do work but they do not kill someone instantly. Some may even last for several years only making the person mentally and/or physically ill.
A death spell can work in many different ways. For example, one can start having suicidal thoughts but just pointing out that not every suicidal thought means you're cursed.
However, a writing, no matter how charged it is, can't kill someone by itself. Then many people would be dead by receiving several "death notes" from their enemies. It's not that easy.
A book cannot kill you, that's true. A negative book which may contain dark arts and baneful magick not only doesn't kill you, but will charge your area for your further rituals. Knowledge never hurts, words do not kill and books are our friends. Not little giant monsters :)
So, death spells and death notes are different. Words do not possess the ability to kill someone that easily.