There is very little about it, unfortunatley.
One study showed, that breastfeeding toddlers did not get as many ear infections as those weaned or formula fed.
One book I read said that not one single case has ever occurred where a child was harmed from breastfeeding. There is not one single case in psychology or psychiatry where a child was traumatized or damaged from breastfeeding into childhood.
One case study followed the children of mothers who in times of famine breastfed children, or provided breastmilk for children, as old as 12 years old to keep them from starving. These moms had fewer children die of malnutrition and disease. All the children, when grown, reported they were grateful to their mothers and were not damaged, but actually benefitted from, the extended breastfeeding. One grown child said that while some mothers were practicing infanticide to save their children from starvation, his mother was nursing all 5 of hers, and they all survived.
I wish I could remember the name of the book or something more solid for you.