May I recommend
Norman Monath's "How to Play Popular Piano in 10 Easy Lessons: The Fastest, Easiest Way to Learn to Play from Sheet Music or by Ear"?
My father told me, for the piano capability I wanted (high-yield learning, low expectations beyond a wee sing song), learning to read a fake book would be the optimal path. He was right, and subsequently a fee of my friends have learnt from the same book! There are usually some cheap copies on AbeBooks.com