here are some of the garter books i have come across, there are some more about the place but i cant lay my hands on them at the moment.
one of them i cant find is breeding snakes in captivity it is from about the 1970's and would be considered very non pc as it details some of his experiments in garter crosses the author is Robert riches if i remember correctly, and he also deals with natrix and if i remember correctly he also crosses them, writing this has whetted my appetite to read this book again so i shall probably spend a large portion of the next few days looking for it.