I'm developing something with the Google Books API, but I think this can be generalized across many of Google's APIs.
Suppose I have an app that's got a social aspect where I can add friends and see their Google Books collection. From the API docs, it says I can retrieve anybody's collection if I know their userid, but I can't find any way to retrieve it programmatically. The only thing I can expect a user to know is their friend's gmail address (or Google Account Login, which, for these purposes, is the same thing). The only way I can somehow get it is through a URL on their web interface for Google books. Making users do it that way is an obscure form of torture.
Does anyone know if this is possible with any Google service?
Specifically, suppose I had a gmail address: example@gmail.com, could I then query some Google service to get the userid for that user? A userid is a basically a really big number (around 20 digits)