So first kc_heath asked about the difficulties of selling a series, based on one of the articles I listed in this post. I'm not zipping off to reread and I was semi-brain dead when reading them the first time, so I'm just going to say what I think of series selling, though my guess is that she read it on Shanna Swenson's blog (shanna_s).
I think in fantasy (and I'd include all forms of fantasy including paranormal romance), that it's easier to sell a series (or trilogy) rather than not. But back up a step. I'd say easier to sell the first three books than not. Part of that is because publishers are looking for trilogies and series. They like them because they can build readers as the story progresses. That means your names gets established and your sales increase with each book (footnote on that below). They really don't want a one book author that much, because they want to build your name and build your books so you make them regular money. Makes perfect sense.
They are willing to risk usually a two or three book deal on a new author and a lot of that has to do with building momentum and timing. Think of it this way. You write a book. You submit it and it is accepted. Now that book won't be released for a year (usually--the exceptions are that recently publishers have been doing rapid release for some authors--Naomi Novik and Jennifer Stevenson are just two--where they try to release the first books of a series or a trilogy within a few months of each other.) So while your book is waiting to be published, guess what you're doing? Writing the next one. And it won't be turned in usually until right around the time that your first book is published. And they won't have any concrete sales numbers for at least six months--halfway to when your next book will hit the shelves.
What this means is that because of timing, they are willing to take a chance that your books will take off and so have your next book in the pipeline to achieve momentum. If they only bought one and waited to see how it did, then the second book might not hit the shelves for a minimum of two years after the first one, and people have forgotten your name.
So you can see that it's in a publisher's best interests to buy 2-3 books. But they usually don't go for more until they at least see the numbers for the first book. If you hit the bestseller charts, then chances are they want to lock you in and will come offering a contract for some more books in the series--Patti Briggs' Mercy series anyone? But most of the time they'll wait until you turn in the third book and can have more solid numbers.
Now, if they look at the first three books in a five book series and think the numbers are meh, they might offer for one more book to see. They might ask for something else from you all together and drop any idea of any more books in that series. Which is painful.
So it's probably better to plan a three book arc (trilogy) for your first books, than plan a series. If I had to guess, I would say that that is why Mercedes Lackey's Valdemar books were usually in threes--because that's what the publisher would commit to.
Now the footnote on usually sales build: The truth is that most often the first book in a series sells better than the next books because it is on the shelves longer, and because some people will pick up a first book but decide not to read the next books. That isn't universally true, but often occurs.
And now a word about numbers. The ability for authors to sell books to publishers is based entirely on the numbers. Sometimes they look at sell through (what percentage of books sold compared to how many shipped). Sometimes they look at other metrics. But when it comes down to it, if you aren't selling books, then bookstores don't order them in good numbers, so that you don't sell books and so bookstores don't order as many, and so you have the slow death spiral/slow dwindle of death.
Now that the economy is looking so ugly with a gallon of gas so much, people will probably back of buying books. That's bad because there are authors who will not be contracted again because of it. So if you can, buy new books (used books don't count at all as far as numbers--authors and publishers don't get paid for them). Recommend favorite authors everywhere you can. Get your local bookstores to order in their books and keep them on the shelves. Tell booksellers how great the books are so they will handsell them (recommend them to customers). Ask your library to order them. Blog about them. Keep telling people.
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