I am also self-taught, but what helped me was playing around with the music. For example, when a piece had "8va" on it I thought logically that it had to do with playing the written note differently; I played it 1 octave higher and it was correct. Playing songs that you know or have a recording of really helps.
This website helped me, too. If you scroll down to the bottom of this page, you will see the heading "Other Stuff". Click on "Music Glossary" for definitions on different musical terms or click "Music Theory" for some of the basics.
I appologize, though, that I never used a piano book that teaches you piano; instead I bought a book that's claims it's for "players of all levels". I thought it would be fun because I know almost all of the songs in it. It's called "Piano Classics", printed by Main Street. (Here's the URL if you're interested: http://www.sterlingpublishing.com/catalog?isbn=9780760770115)
Sorry, the URL didn't go directly to the page. If you want to see it, type "Piano Classics" in the upper tight-hand corner under "Quick Search".