And yes, this is really true.
The history behind the book is, as you might expect, quite interesting. This is not a new novel that she has just completed. Rather, Go Set a Watchman was written fairly contemporaneously with To Kill a Mockinbird, in the 1950s. In fact Ms. Lee wrote it before To Kill a Mockingbird and is actually a sort of sequel, however she put it aside and it became lost over the decades..
"In the mid-1950s, I completed a novel called 'Go Set a Watchman,'" she said in a statement from the book's publisher, the appropriately-named Harper, an imprint of Harper Collins. "It features the character known as Scout as an adult woman, and I thought it a pretty decent effort. My editor, who was taken by the flashbacks to Scout's childhood, persuaded me to write a novel (what became 'To Kill a Mockingbird') from the point of view of the young Scout. I was a first-time writer, so I did as I was told. I hadn't realized it (the original book) had survived, so was surprised and delighted when my dear friend and lawyer Tonja Carter discovered it. After much thought and hesitation, I shared it with a handful of people I trust and was pleased to hear that they considered it worthy of publication. I am humbled and amazed that this will now be published after all these years."
By the way, great and fascinating as the publishing news is, the wonderful news I was referring to above is knowing that Harper Lee is still alive. And is still around to see the book published.
"Go Set a Watchman is set during the mid-1950s and features many of the characters from To Kill a Mockingbird some twenty years later. Scout (Jean Louise Finch) has returned to Maycomb from New York to visit her father, Atticus. She is forced to grapple with issues both personal and political as she tries to understand her father's attitude toward society, and her own feelings about the place where she was born and spent her childhood."
So, the next time sometime tells you to quit procrastinating, you now have a patron saint to point to. Though it helps to have actually done the work half a century before...