This instructable flows out of Rayeath's very helpful nook cover instructable with some (hopefully) helpful changes.

How to convert a hardback book into a Kindle cover with a strap:

What you will need:
1) A book you don't mind chopping up; this book must be at least 7.5" tall, 5" wide, and 1 cm thick (though a little thicker might be better)
2) Two sheets of craft felt which are larger than the books covers by approximately 2 inches on all sides
3) Hot glue gun & glue sticks
4) Up to 3' of 1/2" braided elastic
5) Stiff paperboard; Rayeath recommends a cereal box, I used the covers of an old composition notebook
6) Scissors
7) X-acto knife
8) Paperclip
9) Ruler
10) Sharpie

I spent less than $10 on this whole project, including the book, which I found for 99 cents at Goodwill. It took me about two hours, but this is my first attempt, as well as my first instructable, so a more competent crafter could move more quickly, I imagine.

Step 1: Prep the Book

Using the X-acto blade, cut in the seam of the front interior and back interior of the book.

After removing all of the bound paper, the book should be able to lay flat.

Cut a piece of felt to stretch across the binding and hot glue it to the spine of the book.

Step 2: Prep the Boards

Next, cut the paperboard to match the cover of the book.

After this, lay the board over the felt, and cut a 45 degree corner from the felt on each corner.

I recommend also snipping a tiny 45 degree section from each corner of the paperboard--this helped me a lot by eliminating a pressure point that looked as though it might push through at some future date.

Fold the flaps down and hot glue into place.

Step 3: Install the Corner Straps

Now for the real fun!

Cut four 4" strips of elastic and prepare a sturdy paperclip to pull the elastic through the cuts you are about to make. The general shape should be an eyelet, with enough space left to grip the paperclip.

Center your Kindle (or other e-reading device) on the back of one of the prepared pieces of paperboard, and trace its outline. (I had to use a Sharpie to mark the felt.)

Now measure in from the corner of this traced outline 1" in each direction, and punch a hole through the felt and paperboard at this spot. Cut this hole wide enough for the elastic, but be careful not to make it too wide.

Next push the eyelet of the paperclip through the hole from the back of the paperboard, and load it with an end of the elastic strip. Pull this through (you might need pliers for the grip), and repeat on the other end. You will need to do this for each corner.

You might want to dry-fit your e-reader before the next step to get an idea of how much slack the elastic should have on the front of the paperboard. For the Kindle 3, I needed no slack.

Now, hot-glue the ends to the back of the paperboard.

Step 4: Install the Cover Strap

Next, measure 1 inch up and 1 inch in on the back cover corners, and cut an opening wide enough for the elastic strip.

Pull the end of the strip through this opening as you did with the paperboard. Hot glue the first end.

Now dry fit the cover with the reader and paperboard settled inside, and test the length of the cover strap. Cut to the desired length, pull through the cover, and hot glue the second end.

Step 5: Finish It Off

Hot glue the paperboards into the cover. You'll need to move quickly and make sure the glue is very hot so you have time to work with it. Line everything up neatly, press it in place, and you're nearly done.

Final step: Insert e-reader, turn on, enjoy.