This article from The Guardian contains an impressive mix of information and views on transracial adoption. The article appeared more than two and a half years ago, though it's the best thing I've found on the matter since I saw The Guardian's interview with Barnado's outgoing chief exec, Martin Narey, this week.
Narey criticised local authorities and adoption agencies for preventing adoptions on the grounds that potential parents' ethnicities don't match the child's. The ensuing delays -- and in some cases even relocations -- has led many willing parents to give up, while the children -- predominantly black, Asian or mixed race -- remain in care. Meanwhile adoption is at an all-time low: according to the interview, "[o]nly 70 babies were adopted last year compared with 4,000 in 1976."