I used mat board (mount board) for these, some pieces with a slight texture; I decided that if these sets were made by hand, there should be fine tool marks. The sets were cut with scissors, for an uneven, handmade look, from 1/4" (less than 1 cm) strips cut with an Exacto knife; I wore out two or three blades!
With this method, sets are laid lengthwise to create a kerb; then the rows are filled in in a staggered pattern.
As I thought I was done, I realized that I couldn't have a straight edge where the book walls meet the sidewalk, so I peeled the last sets in each row off, and replaced them with custom-fitted ones; and I found a pre-painted kerb I just had to saw to fit, so I could shape the granite stones around it.
Now I have to paint them to look like worn granite brick-shaped stones....