The letter is attached here. It gives details of the draft agenda and notice that the current IAU Executive Council stands down at this Congress, Members can be re-elected and IAU Federations are invited to send in nominations for the IAU members and Area representatives. If you want more details get in touch and also contact your Member Federation if you are interested.
For reference the current Constitution which is elsewhere on the website is also attached here.
IAU General Secretary
23rd August 2016