Yahoo is releasing an Address Book API today that will give 3rd-party developers access Yahoo users’ contact lists without the traditional, but primitive, method of page scraping.

In addition to searching for specific contacts and fields and reading their data, developers can use it to add contacts and change existing records (although to start, only pre-approved developers will have the right to make edits).

Chris Yeh, the head of the Yahoo developer network, considers this release the second major “proof point” of Yahoo’s Open Services (YOS) campaign, which kicked off at the Web 2.0 Expo in March. The first point was Search Monkey, which makes it possible for anyone to enhance the way website results are displayed in Yahoo search.

As with Microsoft and Google’s own contact APIs, Yahoo has decided to implement a proprietary permission system - theirs called bbAuth - rather than implement an open protocol like oAuth. Yeh says he hopes to see oAuth adopted by Yahoo in the near term, although he couldn’t say when that might happen.

LinkedIn and Plaxo are two launch partners who have already implemented the new API and even used it publicly over the past several months.

Yeh says there is no policy in place for restricting how long developers can store and use the data they pull from the API. But, as with many of its developer initiatives, Yahoo reserves the right to stop what it deems bad behavior.

As Dave McClure suggested to me recently, it would be very powerful if developers could not only retrieve basic contact information from webmail services like Yahoo, Hotmail and Gmail, but could also determine the types of relationships a user has with those contacts. For example, if I wanted to pull out a user’s top 5 contacts, I could do so by looking at the frequency of messages sent to all contacts. This lookup could be refined by targeting only messages with certain keywords so that contacts belonging to particular categories (say, golf enthusiasts) could be identified by their messages.

Unfortunately, no such advanced querying is available with Yahoo’s new API, at least to start. Yeh does assure me that other groups within the YOS campaign are looking at how to identify relationships within the address book, so hopefully we’ll see this type of functionality down the line.

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