I recently culled my kindle archive. I permanently deleted around 900-1100 books. I started buying kindle books since end of 2008 when I got my first kindle and about what was it, 2 years ago or so there were free books everywhere. I went nuts.
It took me about 3 weeks to go through it all. I had to open several tabs for the my kindle page so it wouldn't refresh each delete, then I had to go through each separate, check the book on amazon, check the book on goodreads, update goodreads shelfs if needed, or delete book from goodreads also. And then I also had to do the same in calibre. For each book. One at a time.
I started with the oldest in the kindle account page and worked my way forward and I did it by month purchased and then checked them off. Now I got it all synchronized between goodreads, calibre and my kindle account. The only thing that throws things off are the bundles.
Most of what I deleted was just really bad. Some were chapter samples, some where authors that don't even exist anymore, some by now had such horrible reviews I didn't even want to look closer and then there were the ones I must have been sleep one clicking on because there was no logical reason I would get such, um things.
I now only get freebies pretty much if they have some sort of track record. Back list titles, authors that have a backlist etc. Or some reviews from trusted online friends. I don't get the maybe's anymore.