Piano for dummies or Idiots guide to Piano. $20 or free if you go to your local library.
really go to your library and check out some books - it's free, you can get by. if you already know guitar and the notes then it should be just fingers to keys practice, you already know what a chord is and that's more than most hobbyists know.
Libraries have DVDs, books, etc for free check out. invaluable resource especially for something like fundamental learning.