About twenty-five years ago, I read the fourth-book of a six-book series. I have been waiting ever since for books five and six. The first four books are out of print and copies are getting hard to find, so I can't really bone up on the story if book five comes out.
I hate this. Over the last quarter-century, my interest in the series has waned, so I'm not sure I would even buy book five if it does come out.
About two years ago, I bought a book touted as being an interesting story about post-apocalyptic civilization. Unfortunately, the author for got to tell the readers (me) that the story was going to span several books, each about 75 pages long. He also forgot to mention that he had only written the first book. Haven't heard from him since.
My take-away? Write the series before releasing the first book. In fact, Jo and I write the complete first arc of the series before releasing all of the books in the arc simultaneously. Look at Engine of Creation or Stryker Team. Engine of Creation is made up of two three-book arcs, one that ends with Engine while the second ends with Resolution. Stryker Team consists of a four-book arc followed by four stand-alone stories. Jo and I have decided that we will never leave our readers hanging. If we write a series, then we will finish it in a timely fashion.