I'm using the Faber Adult Piano Adventures books, and am having a great time. I try and learn one new piece each day (they are short pieces -- at most about 1 to 2 pages). The method books are great because they start from absolute basics and build as you go, so as long as you are honest with yourself and don't skip ahead until ready, they are a great way to learn, especially if you are interested in learning to read sheet music.

Aside from that the biggest help has been listening to a lot of classical piano music and that has also been a rewarding experience on its own.