The kindle app for WP is crap. As of today they have an update. An update to fix a couple of bugs, nothing else I was so disappointed.
The biggest issue I have with it is the lack of search. ie. I have over 600 books and the only way to download from the phone is by author or title. You have to scroll through a single line of 600 books to find the one you want, in the meantime another half dozen have downloaded by mistake.
There is NO dictionary in this app. When you compare the kindle app for ios and android. They have even got cloud collections. Collections I've just to scroll through 600 books.
In my opinion Amazon has forgotten windows anything as the windows 8 app lacks loads of things as well.
Anyway rant over. I have been emailing Amazon for months now and getting nowhere.
The work around I have been using is by having kindle app open and on computer I log into manage my kindle and choose WP to download books to. It's slightly less frustrating than scrolling through hundreds of books.
The only thing it has going for it is you can read most of your kindle books on it.
As of November 2013 I stopped buying ebooks from Amazon.