This question has been answered again and again by agents, but it just keeps coming up. After answering it yesterday on a writers' loop to which I belong, I got an email today from another member of that loop, asking it again, apparently convinced my first answer didn't apply to him.
The question? How do I write a query for my series?
Here are my Three Rules About Writing a Series:
1) Don't try to query a series. Concentrate on writing one good book and querying that one alone. Until you sell the first book, you don't HAVE a series.
2) The first book of your proposed series must able to stand alone. That means the reader must finish it and feel a sense of satisfaction. You cannot leave your reader hanging. Therefore, do not make the mistake of believing you can divide your 300,000-word epic into a three-book series. The story arc will be off--I guarantee it.
3) Be careful about introducing a lot of extraneous elements into that first book because you're "laying the groundwork" for the other books. If the elements don't further the storyline of the first book, delete them. If you're lucky enough to get a multi-book contract, you can introduce the new material in later books.
My mother always said, "Learn to walk before you run." Otherwise, you risk tripping yourself up.