I’m a bit of a big fan of Alasdair MacIntyre. His Short History of Ethics carried me through my studies at university before reading After Virtue and then moving in a slightly different direction. Here he describes Nietzsche’s criticism of Kant very clearly. I would have this discussed in any IB or A Level essay on Kant’s Ethics:

…Nietzsche’s accusation is that in fact Kant assumes what he sets out to prove. He takes it for granted that we are entitled to make moral judgements and enquires what must be the case if this is so; he never asks, as Nietzsche does, whether we are so entitled.