Bear in mind that readers may come to a sequel months, or even years, after reading the first book, or possibly not in order. Always recap the plot and describe the characters again to refresh their memory as well as assure any new readers don’t feel lost.
I run into this a lot, and have even caught myself guilty of this while writing my series. To the author, it is one continuous story. To the reader, however, who may have read the predecessor ages or at least many books ago, remembering exactly what was going on as well as the function of minor characters, is often unclear.
In some cases, you may want to use a prologue or a “The Story so Far” section for this purpose. Those who don’t need the review can skip it while those who do will appreciate it and allow them to get fully engaged in the story more easily.
Ending a book in a series with a jaw-dropping cliffhanger comes with a risk, especially if the next episode isn’t yet available. There’s a fine line between leaving a reader intrigued versus frustrated. The latter may lose a potential fan.
If the sequel has been released, this is less risky. If not, and you absolutely want to end it that way, consider waiting to release the entire series at the same time. If the ending isn’t so abrupt that it drives readers crazy, then it’s not quite as hazardous to your fan base if they have to wait a while for the next episode.
However, there is nothing more disappointing to a reader than really getting attached to a great story that doesn’t have a satisfying ending. If there’s any doubt you’ll complete the story properly, especially within a reasonable amount of time, then avoid cliffhangers. Use a reasonable, albeit soft ending, you can pick up from later.
When writing a series, be sure to note at the conclusion of each volume (except the final one, of course) that the story will be continued. Include the title and link, if it’s already written, a potential release date otherwise. Without such information, readers may think you just got tired of writing and quit, leaving them frustrated with regard to what happens if you ended with a cliffhanger. If you didn’t, then readers may not realize that the story will be continued. If you know it’s going to be a series when you finish writing the first book, go ahead and put “Volume I” or “Book 1” (or something along those lines) on the cover, another clue for readers that there’s more to come.
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.