Not to knock COPS, but I was very impressed by Calibre Companion's nearly instant access to my library. Hardest part of using it was the wording of the first tutorial screen was a bit odd to me, kind of like LOTR's "speak friend and enter" actually meaning "say the word, "friend", and enter". Anyway, I was downloading books in 10 minutes.
Unfortunately there are issues with regards to using Calibre Companion on a Kindle device.
Calibre Companion FAQ: Why can't I install CC on Kindle android devices?