Which of Tolkiens works describes the populations around and past the Sea of Rhun?

Are there any parts of Tolkiens books that outline in more detail the populations and life East of Rhun? I have seen differences in locations of cities and towns around the Sea of Rhun itself, as well as an Island in the centre that sometimes is omitted in other maps.. I'm wanting to know what is the best and most detailed description of this part of middle earth.