Answering this question requires some self-reflection. At first glance, it seems it would be best to start the book right away–while the memories are still fresh. But nine times out of ten, that actually results in poor writing quality.
If you’re writing about a traumatic experience, it’s vital to ask yourself whether you are still “living your story”. In some sense, as long as you are alive, you are still living your story. But it is possible to look at a trauma and say it is truly over; it no longer dominates your life.
For instance, if you want to write about your experience with a certain medical crisis, it might not be wise to start writing your book a few days after your diagnosis. Book-writing is a demanding commitment, and it can compound your stress load when you’re still living through a difficult event. Revision and editing partnerships require that you are emotionally strong enough to disengage from your writing and receive constructive critique—without feeling that you, as a person, are being criticized. Wow, that’s tricky!
Whenever possible, it’s best to wait 7-10 years before crafting a manuscript about a traumatic experience.
That said, it IS a great idea to keep a journal during a traumatic time. Let yourself freewrite with no need to edit or correct yourself. Don’t worry about the quality of the writing; chances are it will be rough. Regard your journal entries as interviews with yourself. A few years down the road, you’ll put on your journalist hat and analyze the entries in the same way that a reporter goes through interview notes–finding the best quotes and determining the real story line. The journal is NOT your book. It becomes material for the book manuscript—which is a completely different form of writing.