Serial Killers: A series versus a one-off book?

As you probably all know, Diamonds & Dust, which was rejected out of hand by my ex-agent as ''not remotely publishable'' and subsequently went on not only to be published, but to be up for the CWA Historical Dagger, the Walter Scott Prize, the Folio Society Prize, and score 60+ reviews on Amazon, is now developing offspring.

It wasn't meant to. Seriously. Grateful as I was to Crooked Cat Books, I didn't envisage trotting out the two Victorian detectives Stride and Cully again. But like lily pond paintings by Monet and Haydn String Quartets, once started, it seemed logical to keep going.

Thus the sequel, Honour & Obey, which was published last November, and Death & Dominion which will appear this November. I have also started writing Murder & Mayhem which will be the fourth outing for Stride & Cully.

There are those writers who regard a series as a bit of a ''cop-out'': after all, you've got your characters already written for you. To them I would say: writing a series is MUCH harder than producing a one-off text. And I know what I'm talking about: this is my second series of books. (The Spy Girl series for Usborne was the first)

The main problem is that, unless you started with the idea of writing a series, and few authors do, they just tend to evolve, you are stuck with whatever you wrote in the first one. You cannot radically alter the appearance nor personality of the main character/s without readers going ''What the ...?'' After all, it was how they were in book 1 that will keep them reading books 2, 3, 4, 5 etc. You can and must develop the main characters, but in essence, they have to bear some resemblance to how they were in the beginning.

Then there is the problem of keeping the plot momentum going. I find book 2 is usually the easiest, as it seems to evolve naturally out of the first one. Book 3, however, is far more problematic. New areas have to be introduced to keep the reader interested. Some fundamental shifting of perspective must take place, or else book 3 becomes merely a watered down version of the previous two. Actually, book 3 is usually the pivotal one upon which the rest of the series rests. If you cannot pull it off successfully, it is best to admit defeat and pretend you only meant to write two in the first place.

By book 4, the pitfall is over-confidence. You have run the gauntlet of three books. You feel the surge of expertise as fingers hit keyboard. This, after the previous three, will be a doddle to write. You have your characters, you know how the story arc works. Sometimes this attitude pays off: I still think Dead Man Talking, the fourth Spy Girl book, is the best plotted. However, beware: book 4 can so easily wander off into alien territory, or become a repetition of book 3 with added lacklustre.

I have never got further than book 5 (and Usborne turned it down) so I cannot speak from experience, but I can say from avidly reading crime series, that some writers manage to sustain plot, characters and reader interest beyond book 5, but many more don't. The trouble with series is that publishers LOVE them. They are easy to market, and each book sells on the back of the previous ones. Thus the temptation to go on churning them out year after year, when by rights the whole thing should have been allowed to quietly slink off and hide in a dark corner after the fifth one.

I have been told, though, that the ''real money'' comes from a 5 book series, which means most other writers will have been told this too. I can't see myself getting as far as a fifth book right now. Mind, I never thought I'd get as far as a third. In the meantime, I plot on with book 4, crossing my fingers, hoping that it will avoid the ubiquitous plotholes and that I can pull it off successfully yet again.

So what's your experience? Do you prefer a series? Or a one off novel. If you are a writer, have you ever tackled a series, or does the prospect fill you with horror? Do share your thoughts....