First, one must acknowledge ones utter and complete absence from the digiverse. One month off the blogosphere and it feels like a dead-zone!
This here blog, I doth declare, is about the series phenomenon. Okay, not so phenomenal. In fact...sometimes downright scary! For an agent that is. I'm talking about one thing right now: Do we agents want to read that you have written your sixth book in a twelve book series? Don't we want to know that you have a never ending Old Faithful of Imagination?
We agents do NOT want to hear that you have written twelve books and that you are proudly submitting #6. You wanna know why? Because I don't want to have to read Books 1-5 just to understand Book 6. And in most cases it's worse if the other books have been published before (traditional or self).
If they've been published by a traditional publisher before, then it's likely that they were dropped because of sales. Not many publishers are going to want to pick up a book in a series that didn't do well in the first place. "But it didn't get the right marketing!" you might say. And you might be right. No matter, numbers are numbers.
If you've self-published the first five books and Book 6 is knock-my-socks-off stellar, a traditional publisher isn't going to want to have Books 1-5 out there for free while Book 6 is on the shelves at B&N, so really you're back to square one...or Book 1, that is. And maybe Book 1 just kinda stank.
So you're best bet when querying -if you intend for your ms to be part of a series- is to say something like, "Though I have written this book as the first in a series, it can also work as a stand alone novel."