sp :=
sp += # add space
chapter-sort = $(shell echo -e $(subst $(sp),'\n',$2) | sort $1 -)

books : Book1.pdf Book1.epub

Book1.pdf : $(call chapter-sort,-n,$(wildcard Book-1/*.md))
	pandoc $^ -o $@

Book1.epub : $(call chapter-sort,-n,$(wildcard Book-1/*.md))
	pandoc $^ -o $@

clean:
	rm -rf Book1.pdf

.PHONY: books clean