I was having lunch with my publisher, Martin Fletcher of Headline, last week. I was congratulating him on the cover of 'Fire Force', the sequel to 'Death Force', which will be out next year. It establishes a common identity with the first book, whilst being a great cover in itself. "The same, but different," I observed.

It struck me that "the same but different" was a good way of describing how to approach writing a series of books. Increasingly, publishers want series because the characters can be established over time and the writer can build up an audience. But, of course, it poses challenges to the writer. You need to think about your characters and how they care going to develop over four or five books: in my case ten characters which is especially difficult. And then you need to keep the basic structure of the books similar, while also having sufficient variation to make them fresh and exciting.

There's nothing wrong with "the same but different". Mozart wrote 41 symphonies to which that description could be applied, but that doesn't mean that most of them aren't masterpieces. Of course you can take it too far (Take Van Morrison, for example. A genius, as well, but many of his albums could be described as 'the same but, er, the same). The trick is to get the balance right.

I've just started work on 'Shadow Force', the third in the series. As you can see from the title, I'm keeping one word the same…while the other one is different.