Planning or going with the flow

I've been working on the new book and enjoying myself. Its a different kind of book for me. Its a fantasy but not set in that typical medieval type setting we know so well and its not just a swords and monsters love-fest either. It's a murder mystery.

I'm excited by the characters, the setting, the mystery - except I have one big problem. I have no idea who the murderer is. I seem to remember In Rankin once confessing he had no idea who the criminal was in his Rebus books and he had to find out by writing the book but the thought of this scares me a bit.

I've never been very good at planning books. I wish I was - it would probably save me a lot of work in discarded pages and dead ends. However with this one I think I must.