A book map is a tool for the developmental editing stage of revisions.
Photo taken by Chris Lawton from Unsplash.
What does it look like?
So far, most of the editorial book maps that I have seen and created are in an Microsoft Excel sheet. That does not mean that is the only way you can create one. A writer's book map can look like a series of color coded post-it notes with scenes on it for instance.
What does it contain?
An editorial book map contains at least the following elements:
Main Character Arc Development.
Timeline.
Main and Subplot Threads.
Other elements can be:
Side Character Arc Developments.
Deaths.
Romance Meter.
Lore or World Building Notes.
War, Investigation, or Heist Progression.
Why is it important and helpful?
A book map gives the editor and writer narrative distance from the story itself. You can see all of the pieces of your story laid out in a single document without being bogged down by the details. You can see where there are plot holes, character development issues, or exposition dumps on a structural level.
As the writer, you can move the pieces around, revise scenes, and adjust the story to closer to your vision. On the second pass, your editor can evaluate the changes from the first pass and make sure the big picture is clear before moving onto the next stage of editing: line editing.