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MB3 189a: Eating Cake or Taking Pills with Writing on Them on Shabbos (340:3)
Putting wax on a book page as a bookmark is prohibited because it can lead to ממריח.
Breaking a cookie with letters written on it (as opposed to in the cake itself) is asur even if the intent is to eat it, not erase the letters. Writing/cutting letters made with juice is permissible.
Pills with writing on them and opening books with fore edge printing.