Thick book - find a book that will look like it belongs in the bookshelf. Don't put "The Works of Shakespear" in a shelf with electronics books just because it was the right size. It looks out of place. Better to use that book "10,000 Electronic Diagrams" that you never use anymore. You can get really nice thick well built books for free or very cheaply at garage sales, library sales, old book shops or just around the house. Just ask for permission if it's not your book.
A sharp utility knife or single edge razor blade or exactco knife
Band saw (recommended) or coping saw (useable)
6 min. epoxy or thin super glue - I like 6 min. epoxy because it is thick and will stay where you put it and I'm sensitive to the fumes of super glue.
Strong cloth or paper to reinforce the binding if necessary.
Step 1: Removing the Cover
The better book will have a binding that looks like this. The cover of the spine is separate from the actual spine of the book and you can just cut down the hinge line and separate the cover from the pages.
If the cover is not separate from the spine carefully peel it away after you have cut at the hinge line.
Step 2: Half Way There
Step 3: Cover Removed
Step 4: Starting to Cut the Well
If you want square corners you have to cut the straight lines in the top and bottom first then cut a diagonal line so that you can get to the vertical line at the front.
When you are cutting make sure that you put pressure on the top of the stack and push from the bottom of the stack. If you push from the top of the stack the pages will buckle and the cut won't be aligned. And you stand a good chance of cutting off a finger when the stack buckles.
You could do the cuts using a coping saw but it's going to take much longer. To cut with a coping saw you should clamp the pages between two sheets of plywood already cut to the shape you want so that the book holds its shape while you cut. I did not find this necessary using a band saw.
Step 5: Cut Finished
Of course you can and should recycle the cut out portion of the book.
Step 6: Putting It Back Together
Step 7: Cover Back On
If you want a more finished appearance you can paint the interior or glue felt to the inside to cover up the glued edges.