At LonCon last week I had a conversation with a published author about sequels. Now, if you were to believe the internet, when unpublished, you shouldn't really write sequels and should instead work on making the first book a stand alone. According to the internet, it's pointless working on sequels because the first book might not sell well, meaning you'll be left with a backlog of books that no one wants.
But during my discussion, I heard of two book contracts where these particular authors were given deadlines to hand in the sequel to their first book...before the first book had even gone on sale.
Now I know the demand for a second book would probably happen with two stand alone books, but both authors expressed a little bit of angst at having to produce something that would have - to some degree - been finished had they just written the sequels in the first place.
So what's the truth? Is it better to finish that trilogy before pitching? Or is it better to just plan the hell out of the second book and move on? Or are these simply isolated cases? And are most debut authors asked to produce a second book regardless of the sales of the first?
You're missing a key piece of information here: what does the contract say? Most first time authors that I represent get deals with a contract asking for two or three books. Not stand alones at all. SERIES.
So yes, it's good to have that second book well underway when you get a deal.
Here's what I think you've heard and misinterpreted: don't say you have a series in a query letter.
What that means is you focus on querying the book you have in hand. Don't mention it's the first of N more books, because if I don't like this one, I don't care how many more you have.
Most publishers want books (at least in the categories I rep) that can be built into series. They want this cause once they've invested in you, they want readers to come back for more, and More means More of the Same stuff we loved in Book One.
Focus on writing the very best book you can. Query that book. While you're waiting for us to