At school I hear "David, have you fed the levain?" "Sarah, have you fed the levain?" everyday the beast requires feeding. Then you rip a portion off add it to your bread and Bam - instant sour goodness!
I'm using a book that one of my teachers recommended called: "Bread: A Bakers Book of Techniques and Recipes: by a crazy obsessed bread man called Jeffrey Hamelman. Heaps of recipes, lots of great advice. The books geared to both the home bakers as well as professional bakeries, and the portions show it (for instance 23 loaves of roasted garlic levain vs. 2 loaves).
Anyway, everyone meet the beast.