You've probably already done a fair bit of work for Google.  If you've used reCaptcha (on sites like Twitter, Facebook and CheckMyRoute then you have worked for Google already.  reCaptcha is a device used by websites to prove you're human, and not a 'bot' (a program which goes around the internet signing up for things or accessing websites and doing evil things).  reCaptcha gives you two words to read.  One that it knows the answer to, the other it doesn't, or it's a bit confused with.  The word it knows the answer to proves yourself to be human to reCaptcha.  The other is a 'vote' - a word that's been given to several internet users to cast their opinion over.  If you and a few other users give the same answer, you've helped Google by finally getting that word recognised.  Google needs humans to read these words that computers can't, so instead of employing someone in California, it uses you.

It's not a bad thing.  After all, Google is scanning and making searchable millions of books.  And you're doing your bit by inadvertantly working for Google getting them published on the web.