Day Three's book is located north of the secret thieves bar in Vesper on a tiny island. The exact coordinates are 59o 4'N 109o 53'E. You will need a boat to get to the island if you don't have a friendly rune library, or someone isn't there gating. For those on Great Lakes, you may find runes to all three day's books (and further books as they appear) in the Nameless Rune Library which is due south of the Yew moongate (about 2 screens). Look for the blue runebook on the podium.