Cleanup type and more fully explain reason for strange rule

makefile-book

@@ -7,7 +7,8 @@ DOCUMENTOPTIONS += verseindex=true
 .PHONY: book
 book: $(BOOKBASE)
 
-# CaSILE doesn't expect to have to generate this by defaulat
+# CaSILE doesn't expect to have to generate this by default and works allows
+# them to be not-present.
 $(BOOKBASE)-manifest.yml: $(BOOKBASE).yml
 
 $(BOOKBASE).yml: $(LANGUAGE)/$(CCMBOOKCODE).yml