My primary instrument is voice, but I also play and teach piano. Piano is an excellent way to learn to read music and learn music theory. I recommend it for voice students as an added tool to help you become a better musician or for anyone who wants to begin to learn more about the wonderful world of music.
I use the Alfred books.
For adult beginning piano students the Alfred's basic adult book will help you learn how to play at an accelerated pace.
For children Alfred's has a wide range of books for all ages: Piano books
If you are in need of a piano to practice and play on I recommend this site: Piano adoption