Make a book holder like the Pinterest one or circular saw guy below. I used two pieces of wood with dowels connecting them. The wider(2") piece I glued the end of the dowels into and the thinner(1/2") free slides. The wider piece has a notch so I don't hit the clamp with the knife. I put them about a foot apart for large books and magazines. I use Irwin clamps to hold it together in a metal vise. I slice the spine if it is greater than 1" and use 25MM knife to make it more manageable. I secure it with out about 1/8 below the spine and proceed to slice through. Any larger and you will get a wavy spine. Pictures eventually will follow.

Book Vise https://www.pinterest.com/pin/29336...

Can't get this to work even with a plywood blade. https://www.youtube.com/watch?v=-M9...

Irwin: http://www.irwin.com/tools/clamps/o...

Break-off Knife: https://www.homedepot.com/p/DEWALT-...