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I have seen many books where the first letters of the first 4 works in the first sentence of the work spell out the shem Havaya (the Tetragrammaton, yud-kay and vav kay). What was the first Jewish book that made use of this?
We'll go with Double AA's Mishne Torah for now.
Rambam's Mishneh Torah was compiled, according to Wikipedia, " between 1170 and 1180 (4930–4940)."
It begins (Yesodei HaTorah 1:1) "יסוד היסודות ועמוד החכמות."