Breastfeeding Made Simple by Kathleen Kendall-Tacket and Nanch Mohrbacher.
I LOVE LOVE LOVE their explanation about the physiology of the breast and storage capacity. Once you understand that, you can throw out your clock, throw out the band you wear to keep track of which boob you nursed on last, and focus on your baby and his/her needs and how your body is meeting them.
Real quick, they describe how breast storage capacity varies, while babies' feeding needs top out at about 32-36 oz/day by the time they are 3-4 months old. So, if a mom's breasts hold 2 oz each, they have to nurse on both boobs 8x/day to give the baby 32 oz, whereas a mama with 6 oz storage in each boob might feed her child 6x/day from only one breast and still both provide their babies with adequate nutrition and volume. Baby will tell you when he/she is still hungry, and they will stop when they are full, nurse again when they need more.
There's tons more in there, but it's a great, slimmer volume to get moms off to a good start.