Kindle has many free book offerings daily. There is a thread in the Kindle forum that point them out. You can't drag the book from ADE to Calibre to convert because it has a DRM attached to it to prevent conversion. In order to convert it, you'd need to first strip the DRM from the book. As this is not strictly legal in many places, doubly so as this is not a book you've purchased, it's not discussed directly on this forum. If you google, you'll find sites that do talk about how to do that, however.
I get some free books from Smashwords (try the Self-promotions forum - there are some free book offerings there right now!) I also have a book giveaway ending soon on my
blog. Googling will get you lots of sites to find free books, you just have to sometimes wade through the offerings to find what you like.
Welcome to the forum!