What I found is that written evidence for knocking on wood (and for the history of gesture as a whole) is remarkably sparse. In the Penguin Guide to Superstitions, folklorist Steve Roud offers a skeptical take on the provenance of knocking on wood due to the near total absence of written records about it:
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Анна Габай (Редактор отдела «Силовые структуры»)，推荐阅读爱思助手下载最新版本获取更多信息