It has always been my understanding that Kaddish is recited for Ilui Neshama of the niftar, and that only the recital of Kaddish will serve this purpose. Now I have been asked if donating a number of siddurim to a bet knesset in the name of the niftar will also contribute to his ilui neshama.
Reciting kaddish is a very strong illuy neshoma, and therefore the children of the niftar say kaddish. The children cannot say that they will donate siddurim to a Beit Haknesset instead of saying kaddish. However, there are other forms of illuy neshoma, such as learning mishnayos or saying tehillim for the benefit of the niftar, donating books for their merit, or doing a chesed in thier merit. All these things are an illuy neshoma, and a benefit to the niftar’s neshoma.