Actually, you can easily choose which books to download to your device from the Archive (now called Cloud on some devices). I'm not sure if you can do bulk downloading of selected books, but you can certainly do them individually.

Once you delete a book from your device, it remains in the Archive (Cloud) in case you ever want to access it again. You can delete books from the Archive (Cloud) but doing so permanently deletes them which means if you ever want to look at it again you will have to re-purchase it. So be cautious not to delete from the Archive (Cloud) any books you would like to keep stored.