Draw a horizontal rectangle on your piece of paper (accuracy is not important here). The right and left lines represent the height of the book cover and the top and bottom lines represent the distance from the left edge to the right edge of the book when it is opened flat (including the spine).
Take your height measurement and add 1" for top and bottom seam allowances (this means you will sew with 1/2" seam allowance) and some wiggle room (between 1/4" and 1/2" is best). Write the resulting number on the right or left side of your rectangle. Now take your width measurement and thickness measurement and add them together. Add 1" for the side seam allowances (again, sew with 1/2" seam allowance) and more wiggle room. Write this number to the top or bottom of your diagram. This is for the main cover.
For the endpaper pockets, create a second diagram by sketching a vertical rectangle. The height will be the same as for the cover, so just copy that number over to the left or right side of the rectangle. Now, take your width measurement again, cut it in half (give or take), add 1" for a hem on one side and 1/2" for a seam allowance on the other side. Write this number at the top or bottom of the diagram.
Once you have all your notes and diagrams in order, it is time to get to cutting your fabric. Cut out two of the cover pieces and two of the pocket pieces.