Wisdom and knowledge shall be the stability of thy times; literally, and the stability of thy times shall be (i.e. consist in) a rich store of salvations, wisdom, and knowledge. The prophet here addresses the people of Judah in the second person, though in the next clause he reverts to the third.
Such transitions are common in ancient compositions, and especially characterize the writings of Isaiah. The fear of the Lord is his treasure; i.e. the wisdom intended is that which is based upon "the fear of the Lord" (Psalms 111:10).
This will be at once Judah's "treasure," and a guarantee of stability to her government and institutions (compare the Homiletics on Isaiah 32:15-17).