Affirmed by meetings of both the Lambeth Conference and the Anglican Consultative Council the Five Marks of Mission of the Anglican Communion is the nearest thing the Anglican Communion has to an ‘official’ statement about mission. The statement of the Five Marks – and a brief commentary on them – can be found here.
The documents produced by the three recent Mission Commissions of the Anglican Communion, IASCOME; MISSIO and MISAG II also offer useful resources for serious study of Anglican mission thinking
The World Council of Churches website has some useful online statements and articles linked to mission. Of particular interest may be this statement about the link between mission and evangelism in unity today and a discussion about the concept of reconciliation as mission.
There are a number of resources available via the website of the International Association for Mission Studies (IAMS). The Church Mission Society website has many valuable resources on mission.