California Madness is the prequel to Book 5, The Falling Star Recovery, and covers the time in between Book 4, Dragon Fire Recovery and Falling Star.
The reason it’s not included in Book 5 is that it contains what some fans of the series describe as “relationship shit” and that was the polite description. Some readers just aren’t interested in the interpersonal aspects and, since it was a little long, the decision was made to offer it as a free supplement instead of trying to wedge it into Book 5. That way the people interested in the characters personal development can read it and those who prefer to stick to the tech and action can skip it. It also explains Amber’s sudden weight gain before the events in Falling Star and helps explain a significant surprise at the end of Book 5.