I read this written about elsewhere, and it’s a concept that makes sense for me:
When you’re writing a trilogy, you want there to be sub-plots that match up from one book to the next one. Also, you don’t want to make your readers wait a year or more for the next book. To solve this problem, you should write at least the first drafts of other books in a series at the same time.
I’m facing this situation: I’ve been writing the first book of a trilogy (within an overall series) for over two years. If I’m fortunate, I’ll have it finished this year (revisions, beta readers, cover art and editing). With this current pace, I’ll have this first trilogy finished before I’m sixty. That’s not acceptable to me.
That passage of time is another problem for me. I’m 49 in less than two months. I want to be known for a lot of books by the end of my life. I already have 11 RPG credits so far, but most of them are very small press, and that’s a small interest pool. Novels are in a much larger one. I hope to follow of game writers who also became novelists, like Mike Stackpole, the late John Ford, and an also-late friend Aaron Alston.
I won’t achieve that goal at my current output rate. Writing multiple books at once might, depending on my writing output speed.
A caveat, though: I won’t attempt this until after I’ve finished book 1. Friends bought into my attempt to crowdsource that book on Indiegogo. Even though it didn’t make the funding target I want to finish it for them (I used the platform’s Flexible Funding plan, and the rules for that are that I have to finish, but I planned to anyway). Starting other books before I finish this one wouldn’t be fair. Besides, I follow one of Robert Heinlein’s Writing Rules: always finish what you start.
After I finish Book 1, 2 and 3 I’ll work on together, and I’ll hope this makes them more consistent and my output grows.