The Constitutional Arrangements Committee has established its own website to encourage public participation in the process of investigating our constitutional structure and recommending a pathway for change. It can be found at http://www.constitutional.parliament.govt.nz.
So far there's a couple of discussion documents up, as well as a list of suggested reading. While this stuff is useful, it's not essential; constitutional change is about what we want, and you don't need to digest a massive pile of reading in order to have an opinion on that.
The committee also looks like it will recommend that any major change be preceeded by a referendum, which is a good move. Any significant change, such as the move to a republic or written constitution, must be ultimately founded on popular consent. Without that, we're not deciding our own constitution; other people are deciding it for us.