My second book came out a couple of weeks ago. I had a number of signings arranged and expected one or two previous buyers to turn up. But the effect of having the sequel to sell is dynamic. I'm amazed at the number of people, especial YA, who say they prefer to buy two books because they like a series of stories. I'm now under pressure to finish the third book. So for the writers who want to know what to do when the first book has been written, definitely get working on the second one.
