Cain's House is a location in Act I of Diablo III. The house can be found in southern New Tristram. It is inhabited by Deckard Cain, Leah's uncle. Both parts of Deckard Cain's Journal can be found here.
Cain's House can be accessed at any point during the first parts of Act I, but is closed off for good at the end of The Doom of Wortham quest after Cain's death at the hands of Maghda.