1.) Two different items - because the same item is being offered by two different sellers. The $9 is Amazon's main one, the $12 is Macmillan's own specified price (note the "Sold by Macmillan" in that entry).
2.) Prices on the same item (identified by ASIN) do change at Amazon: sellers can change their prices, even to ridiculous lengths such as this from last year: http://www.michaeleisen.org/blog/?p=358 . In the retail part of Amazon, Kindle books are treated in much the same way as physical items, so the same flexibility of pricing is offered. Physical goods have cost of holding stock and 90-day-invoicing cycles, hence pricing seem to fluctuate based on that 90 day window and items still on hand at payment day. Kindle ebooks, less so, but there's still A/B testing to consider.
Online, especially for non-physical items are tending to have fluctuating prices, trying to find the ideal price to maximise profits.
One of my friends recommends http://uk.camelcamelcamel.com/ for price watching items on Amazon. I tend to keep an eye on the Lego-focused variant over at http://brickset.com/buy/uk/amazon/ -- price watching is proving to be an interesting cottage-industry.
Clearing cookies / incognito mode triggers re-selecting a different bucket - that's why you are seeing different pricing. So, if you want, clear cookies over and over, find the lowest price, and buy right there and then.
Interestingly, the counter-intuitive strategy of increasing price can sometimes actually reap more sales. So having the lowest price isn't always the most optimal strategy.
I've had worse price differentials with British Airways. Go directly to the site for a flight and I get one price. Go to Expedia pick the same flight, get redirected straight through to the same page as before and the exact flight is £200 cheaper.