The Google People API
Google is taking steps to replace the Google+ API and the Google Contacts API with the release of its new People API.
The new API will enable developers to pull connection data from authenticated Google users with a single call (rather than multiple calls). Not only does this eliminate the need to use two seperate APIs, there is more information available, and that's sparking the interest of both mobile and web developers.
The People API provides an opportunity for application developers to access users' private contact lists (with permission of course) and determine if contacts are linked to other public profiles. Essentially, the results returned via the API can be used to collect more data about the person and their connections (phone, email, birthday, etc.) - connecting the virtual dots if you will about the actual relationships that exist between Google users.
Google explained in a post on its developer blog:
For example, if your user has contacts in her private contact list, a call to the API (if she provides consent to do so) will retrieve a list containing the contacts merged with any linked profiles. If the user grants the relevant scopes, the results are returned as a people.connections.list object. Each person object in this list will have a resourceName property, which can be used to get additional data about that person with a call to people.get.