Tolkien eventually did write a full account of how Isildur lost the Ring, but it didn't reach the public until Christopher Tolkien published the Unfinished Tales.  Here's the story.  

The Unfinished Tales can be found here: https://www.amazon.com/Unfinished-Tales-Lost-Lore-Middle-earth/dp/0345357116  

Video on the Kingdom of Arnor: https://www.youtube.com/watch?v=6t0zfrkinqc  

Oh, and that awesome map of Middle Earth?  Get it (with 3 others) here: https://www.amazon.com/Maps-Tolkiens-Middle-earth-Brian-Sibley/dp/061839110X/ref=sr_1_2?ie=UTF8&qid=1493809054&sr=8-2&keywords=maps+of+middle+earth