When writing a series, it’s helpful to go back and reread the previous stories before starting the next one. You’ll be surprised how many little details you can tie in or use to create new plot twists. Fans love it when they encounter and recognize such connections, which make them feel like an insider.
Rereading also helps you regain momentum established in the previous story, especially if it’s been a while since you wrote it. Being consistent with details is essential, such as character eye color, relationships, location descriptions, and so forth. Don’t ever assume no one will notice because you don’t even remember yourself. If a reader binge-reads the series later or has a steel-trap memory, you’re going to hear about it, probably in a less than friendly manner. Thus, it’s to your advantage to take the time to do it right. Your fans will appreciate it.