In this pattern, A= book width and height (book based on a square) and C= spine width. Again, a solid line indicates a folded line and a broken line indicates a cutting line. The extensions are cut to the same length and width as the spine, and are optional, but useful to glue down to reinforce the base and spine areas. You can reinforce the sides by completing the square as for a square book, making the diagonal line a fold line and gluing down the fold-in triangle on both sides.
A similar slipcase is one for a quarter circle book (again based on folded squares). Instead of cutting a diagonal line between the line where the base of the book will sit and the top of the slipcase, you cut a curved line or whichever shape it might be that fits your book.
The next post will be about making the unusual book box for That Unbearable Lightness.