No experience of a PC app but I do have Kindle for iPad, I can buy any Kindle book through Amazon and read it on the iPad, I would imagine it will be the same for the PC. When you download the app it will ask you to register it to your amazon account, then away you go. Might be a pain reading a whole book on the PC though.
It will definitely do that Pasta, perhaps download some free books so you are not actually laying out a penny. It's a good way to see if you'd use a Kindle without going to the expense, hope you enjoy.
I used kindle for pc long before I got a kindle (thank you daughter of mine!)
and I found when I plugged it into the laptop it downloaded all the books I'd downloaded onto the pc onto the kindle itself. It wasn't too much of a bother reading books on the laptop either, though the kindle is more portable than the laptop.
I have kindle on my laptop. It was easy to download and when you want a book it is there instantly. I would recommend it to anybody, and it doesn't cost a penny to have it on your laptop, its free from Amazon. And there are lots of free books and others that only cost pennies.