Harpoon rules are in Sword and Fist, and reprinted in the Arms and Equipment Guide. FR-Races of Faerun has a variant harpoon, which shares most of the normal harpoon rules faults and shortcomings.
As greenslime pointed out, they are badly leaky and have some rather obvious gaps. I would call them useless, YMMV.
A rather good version of the harpoon with rules can be found in the Iron Kingdom Character Guide (Privateer Press ), which - even if not "official"/canon clearly illustrate how a Harpoon works with regard to restraining its target if succesfully hit, and what other considerations the WotC rules department missed when they did their writeup. Worthwhile read.