Let's take a look at the positive and negative side of things.
This is a big one for new authors. When they start drafting that first book in their potential series, more time is spent setting up things that will happen in later books than focusing the attention on the book that is happening at that moment. The result of this is that we see more of secondary characters and their issues and traumas than we do of the protagonist. These characters really do steal the show and we don't want that.
Do they need to be introduced in the earlier books? Sure! But they have to remain simply the supporting cast to that first book and that is it. Let us learn their personalities and give us some nuggets to work with, but that is it.
The second big negative comes to sales. If the author has spent all of this time building up that 3 book series, or maybe they have actually written the entire three book series, what happens if the first book doesn't sell? That time you spent on the other books is now gone from your life. Please understand I am not just talking about published authors here; I am talking about unpublished authors as well. If you submit to an agent this three book series and we don't like book 1, we are not likely going to say, "Hey, send us the sequel even though we hated the first one."
We have to remember that published authors who do write a series have the added luxury of knowing their established followers might cut them some slack and hold out for that second or third book. They'll look at that weaker first book as "just a bad writing day," and move on. New authors just don't have the flexibility here.
All of these do come from sales. If you can hook that reader in the first one, then you will have them wanting to come back for more. Of course, this does work for non-series books as well. Essentially we are saying, if the readers like your writing in book 1, they will read whatever you throw at them for later books.
As an agent, seeing someone who has carefully planned out a series and who can see things in both the big picture and on the individual level is a great thing. This means we have someone who probably has a lot of productivity in them and we can look to the future and really think through their career.
I really do recommend for a lot of newer authors to think really one book at a time. Keep those books at a "stand-alone" level for as long as you can, but....
- Know where the story could go if there is a chance for a follow up.
- Think of the books as a common "theme" and not so much the same setting or characters
- Remember that your authors might not be reading the books in order so keep things flexible
- Let the series come naturally and don't force it.