Why is Torah sometimes written in the Torah with a "ת" at the end instead of an "ה"?
As Gesenius writes in his Hebrew Grammar:
(b) The original ־ַת is regularly retained as the feminine termination in the construct state sing. of those nouns which in the absolute state end in ־ָה, e.g. מַלְכָּה queen, מַלְכַּת שְׁבָא the queen of Sheba. But the feminine endings ־֫ ־ֶת, ־֫ ־ַת, and also the plural ־וֹת, remain unchanged in the construct state.