I would always recommend any of the Nancy Clark or Jane Brody nutrition books. Very balanced and healthy. I have not seen anything specific to breastfeeding, but I would imagine a healthy, carb focused approach such as these two advocate would be good. Maybe take a look at the recommendations for pregnant athletes.
My cycling partner is in your situation - it has been tough for her to get back to riding and doing the long rides she's used to (we trained for double centuries together). Her son's still a little younger than yours. I think she may try a century next month, but it's one where you could easily do 70 miles instead, so there is a bail out at least.
Good luck. I know it's probably a sacrifice not being able to put the time in the saddle you may like, but it's a once in a lifetime thing so have fun with your baby. (Once in a lifetime for that particular baby if you have more than one. Once in a lifetime, period, for me! . )