The final vowel may be short, but don't you think there is a very clear difference in pronunciation, in modern English, between my mistress's books and, if we were to say it, my mistress books.
My question was about pronunciation, not spelling.
Still, you answered it. It seems that your pronunciation of this word is different from mine. I would not claim that I would pronounce three clear syllables in mistress's, but my mistress's is very different from mistress