Recently, I tried transferring some new ebooks to my Kindle Paperwhite (first generation). The books were listed properly. However, when I tried to open them, I got the “Unable to Open Item” error, suggesting that I re-download the books from Amazon. I tried transferring the files again and again, but it didn’t help. Some of the books were mobi files, while others were AZW (which I got from אינדיבוק), and all of them opened fine on my computer.
Finally, I followed advice from a comment on the KindledFans blog and converted the files to AZW3 (the original comment suggested mobi, but AZW3 works better with Hebrew). After converting, I moved the files to my Kindle, and they opened just fine.