I'm about to buy a new ebook. I wonder whether to get a Kindle Paperwhite ($159) or the new Kindle Fire ($199). Could someone tell me the reasons for buying one rather than the other?

Well, Kindle Fire HD($199) is a tablet and Paperwhite is a pure reader.
If you mostly read eBooks and seldom (or never) play games or things like that,you can take Paperwhite.

Or I would recommend Kindle Fire HD for you.

However,as the Nexus 7 is hot at present,you may be interested in which one is better,Kindle Fire HD or Nexus 7.
http://www.epubor.com/kindle-fire-hd-vs-google-nexus-7.html
This guide will be helpful to you.

Thanks, AdaWang

Kindle Fire HD is better because it is not only a reader. It is a tablet that has more and maybe better features. I have the Kindle Fire HD and really like it. I really like the Android OS and also trying to develop an own app for it (sg. Like the followings: Ozeki C# VoIP SDK - A SIP SDK for software developers (http://www.voip-sip-sdk.com/p_457-voip-iphone-integration-voip.html)). I would suggest the Kindle Fire HD.