Reddit mentions of The Logic of Subchapter K: A Conceptual Guide to the Taxation of Partnerships (American Casebook Series)

I would also advise you to be aware of what's going through their capital accounts. For a partnership there are technically three sets of capital accounts: 704(b) ('book' per the code), book per the books (GAAP ideally), and then tax capital. Those could potentially all be different, especially if it was property that was contributed to start the partnership. A partnership is basically a big exercise in tracking capital accounts.

As for what you need, as Hitemup27 said, you'll need a set of financials. Last year's return is also reasonable to ask for. That will give you the book sharing ratios, but you absolutely want the partnership agreement as well. That agreement is what dictates how the partnership operates and will give you your TAX sharing ratios. Those are the two most important pieces of information in my mind.

A partnership is a really complex tax beast, arguable the MOST complicated single return type (i.e. not a crazy corporate consolidation). There is a great book out there called the Logic of Subchaper K (http://www.amazon.com/The-Logic-Subchapter-Conceptual-Partnerships/dp/0314233644) that will help with understanding how a partnership operates.

Good luck!