It seems this is the way for amazon to get around the pre-order price guarantee. Cancel your order, make the item unavailable, readvertise the item as a different listing, and bump up the price by several pounds. :(
Yep. Same thing happened to me. Ordered September 2011, at a nice price of £4.88 with a pre-order guarantee.
Now the price has just been "reduced to" £6.80.
I like the series, it isn't brilliant, but it is quite good. I feel annoyed that I've got all the preceding books - mass market paperbacks - and suddenly it isn't a mass market paperback price. But there are apparently only 2 more in the series, this and the next/last one. Damn.
I'll have to buy it, but I'm really, really cross about it.