It’s very common now for writers to treat their manuscript as being the first in a series. That’s fine. Publishers like series books because if the character becomes popular a sort of brand is established and readers look forward to future instalments – which is good for sales.
But for unpublished writers there are two things to beware of.
The lesser and more debatable one is not to lay the whole ‘series’ thing on too thickly when making a submission – you haven’t even had this one accepted yet so it can come across as being presumptuous! My own personal method is to approach it in an understated way: mention in the covering letter the fact that something is written with subsequent stories based on the same character in mind, and that you have ideas for subsequent books should they like this one.
But by far the biggest pitfall I’ve come across with inexperienced writers embarking on a series is that they get so caught up in looking beyond the novel they’re actually submitting that they don’t even give it a proper ending. Instead of bringing the story to a proper conclusion it is left hanging, like an instalment in a weekly series or something: Find out what happens to our hero in the next exciting episode! But this is so irritating for the poor old reader (particularly since it will probably be a year or more before Book 2 comes out!)
Even in a book of a series with themes that are carried over into subsequent stories, each individual novel must be a self-contained story, with a beginning, middle and satisfying ending. That doesn’t mean you can’t leave unfinished business, a sense of ‘We’ve won this battle – but the war isn’t over…’ It’s a matter of getting the balance right, and the worst thing is to put your hero into a particular situation and lead the reader into thinking they are going to find out how he gets out of it at the end – only to get to the last page and find that there is no end!
Not everybody reads every book in a series, and in theory a reader ought to be able to enjoy Book 2 in its own right even if they come across it and get drawn into it having never read Book 1.
The usual disclaimer – for every rule there are exceptions. But we put a lot of blood, sweat and tears into our writing, and the important thing for an unpublished writer is to ask yourself: do I want to bet all my money on a rank outsider, or play it safe? (Okay, I’m mixing my metaphors a bit but you know what I mean!)