This sounds very much like my foray into online poker. I played play money SnGs on Zynga/Facebook poker for a long time before the thought of depositing real money onto a poker site
even occurred to me.
Looking back, I'd say my play money experience did very little to train any actual poker skill. I was "winning" at the "highest" stakes, but only realized later, after I'd moved to real money games, that it was in spite of the fact that I didn't have any clue what I was doing. Neither does anyone else at the play money tables, for that matter. For sure, the quality of play on even the lowest stakes real money SnGs is better than on the highest stakes play money SnGs.
However, there's no harm in dabbling in play money until you feel comfortable enough to risk real money. One thing to which I do credit my (perhaps) overstay at the play money tables (and more generally, my cautious approach to risking money) is that, rather than going busto with it, I was able to run with my first deposit. Some others will more readily play for real money from the get go, but may risk losing several deposits in the process.
The best SnG book out there is almost undoubtedly Collin Moshman's Sit 'n Go Strategy