Since there is no thread for this over in props...
It shouldn't be too difficult. First, use a hardcover book. Something with a fabric wrapped cover.
Just open the book, figure out where the middle page is, and put a bookmark in the middle.
Lay the book flat and then start gluing the pages together. First the first sheet to the front cover, the second sheet to the first, etc. Alternate between the front and back halves of the book. No need to be heavy handed, just few dots of glue about an inch in from the outer edge of the page. Once you're all done, hit it with a few coats polyurethane preservative. Virtually all of the book will be natural plant fibers, so it will soak up the poly nicely, much like a piece of fine furniture. This will also seal all the pages together into a single block.
Once that is all done, I would lay it on a sheet of thin birch plywood that has been cut down to provide a base with a bit of a lip (1/2 to 1 inch wide lip) around the outside. You can get birch plywood in 2x2 foot panels for $8-$10 instead of shelling out $50+ for a 4x8 foot sheet. You want 3/8 inch thickness or less. I've found it as thin as 3/16 inch. You should hit the board with a couple coats of polyurethane as well.
Anyway, carefully center the book and glue it in place by the spine and outer edges of the cover. This will inevitable leave some gaps, as the cover should not have laid completely flat unless someone broke the spine. Gingerly spray some great stuff into the gaps between the cover and board to seal those spaces. If the foam extrudes, use a knife or other straight edge to remove the excess before it sets. I'm fond of using dollar store rules which come 4 to 6 per buck.
Once this is all done, you can attach it to the tombstone. Because the book will be heavy compared to the rest of the tombstone, I suggest using long bolts to fix the board to the tombstone, one through each corner of the board and then through the tombstone with the nuts at the back to hide them from view. Be sure to include locking washers.
After the book is attached, then you'll want to slather on the monster mud to put an unbroken seal over everything and finish the tombstone however you normally would.