You know the old saying.
It isn't just sound egg-maintenance theory. It also applies to many things in the writing life. Including writing & querying series.
Lots of unpublished writers query and say something like: "This is the first book in a series. Books 2 and 3 are complete, I am working on book 4 now!"
This makes me sigh. I read that and see a person who is stuck completely on one story, who is not ready to be flexible and diversify, learn and grow. Not to rain on your parade, but... what if Book 1 is actually fundamentally flawed and you are building a house of sequels on a shoddy foundation? What if it never finds a home? Then all the energy that you spent on sequels is wasted, when you could have been off finding more stories and inventing even more awesome worlds.
The other day a very nice Twitterer inquired during #AskAgent something like (paraphrasing): "I've had book one out on submission for some time... when should I start querying agents book 2?"
Not to be mean, really, but what's the point? Nobody can take on and sell JUST book 2 if it has to be a series. And nobody has picked up book 1. Sooo....
"Well, I'll just self-publish then!"
That's your call, and might be the right path to take. But if you do, don't expect to then query books 3 and 4 and nab an agent to be published traditionally, unless the first two books have been phenomenally successful.
Look, you're an unpublished writer who wants to find an agent for a series, trilogy, duology, sextet, or whatever? (Keep in mind I am ONLY talking about traditional publishing here) - This is what I'd love to see:
A strong book one that stands alone, but where there is series potential. (That means: No huge loose ends in the plot, but perhaps some generally unresolved big picture stuff. Strong, compelling characters that we want to follow.)
A synopsis for book two, and/or an overview of the "series arc" -- that is to say, ideas for future adventures of these people and a vague idea of what might happen in those books. One to a few pages - no need to go overboard. (If you've got really great ideas for Book 2 and just can't wait to put them on paper, go ahead - but no need to show them to an agent at this point.)
Then, to be blunt, you should MOVE ON and write something entirely different for your next project.
Why? Well, if book one is strong enough to support a series and I want to take it on, I can sell it plus probably future books without your having to write all of them in advance.
If book one is NOT strong enough to support a series, it doesn't matter how many books you write -- if we can't sell book one, the other books are dead unless they are able to totally stand alone. And you've wasted years of your life writing them when you could have been pursuing even more amazing projects.
Some series or trilogies are sold as such. But others are sold as a one-book or two-book deal, and the publisher will want to wait to see how the book is received by readers before committing to further books. At the outset, your agent won't be able to guarantee a multi-book offer, it just doesn't always work out that way.
Not everything HAS to be a series, and you don't get a contract for a series just because you feel like it... for a publisher to keep putting books out, there has to be an audience who has demonstrated a keenness for buying them.
Some books take off and become hugely popular with the reading public, and, I promise you, if that is the case, your publisher WILL WANT more books set in the world you've created. But the fact is... some books will probably be smaller, or may not get the attention you hope. This is reality.
The goal is, ideally, to get you on a path where you write amazing stories and invent wonderful characters as a career. When you are looking for an agent, you want to maximize the chances for one of them to fall in love with your work. Once you get an agent, they'll want to maximize the number of books you sell and stories you tell. So why lock yourself into one world?
Get that story-quiver full of good, solid, arrows - - so if one misses the mark, you'll have others you are excited to string.