Larger casings will case harden in a fridge and could trap spoilage or pathogenic bacteria in the center of the sausage with sufficient moisture for them to multiply. Lamb casings dry quickly to lower the water activity within the sausage. A culture should decrease the pH (an important safety hurdle) and can "crowd out" bad organisms. They can improve color and aroma and some can reduce residual nitrite levels. The book I would recommend is this one:
https://www.amazon.com/Home-Production- ... d+sausages
This one is also good (by the same authors):
https://www.amazon.com/Art-Making-Ferme ... R1PX0KRB4S