There are numerous websites and texts dedicated to the mysteries of Google Recruitment.
It's well worth spending the time to read up on as much as you can, especially as to how to approach them in the first place.
However, as far as I'm aware they do not use or even entertain agencies. They have no problems with spec letters and cvs being sent in though, but do some research just to see the kind of things they look for as they are notoriously picky.
Unfortunately most of the information available is with regards to the tech and engineering jobs. But, they apply the same rigourous and extremely high expectations on all staff, so the information is still relevant.