I had a custom order for a purse with a detachable Kindle Case. Its sounds like a handful at first, but after a few hours of working on it I have it about half done. The case I have completed and it was fairly simple.
I measured the Kindle (this one was a 3rd generation with keyboard) and added about and inch to the length and height for the fabric. I cut two of these rectangles out of the liner fabric and the outer fabric. Facing wrong sides together of each, I sewed three sides, leave one longer side open. I turned the outer piece right side out and slipped the inner piece inside of it. Cutting two pieces of sheet foam (found in the kid's craft section) that would fit in between the liner and outer fabric, I slipped them between the two "pouches". This provides cushion for the Kindle. Once the foam was in place I pinned the zipped on the open edge and sewed it in place.
For this one I added snaps on the back before sewing the zipper in place. You can also add variation like a handle, buttons, or wrist strap.