I like to focus on one of my books as my main project, but I do end up taking periodic side trips to my other stories.
In the back of my thoughts, while writing in my Wild Heritance series, if something I can use for a character or scene in another book takes shape, I’ll divert to make a note, or write a paragraph – or several. And then I return to the main writing project. Although sometimes I get caught up in the other story for a bit, so it might take a couple of days for me to get back to the main one.
This means I’m actually writing several stories simultaneously. I love it that way! If I’ve run into difficulty with part of one story, I can jump to a different one and write there, while working out the problem with the first in the back of my thoughts. It also helps me try to keep everything consistent throughout the series.
This is how I have Wild Heritance Book 1 in its final (I think) edit before publication, the draft for Book 2 at about halfway done, and the Novella in its 4th/5th revision.