Complex sentence: F.B. Pinion agreed to a large extent that "Agnes Grey" was quite a masterwork but that Moore's examination of the piece was a little extreme and his "preoccupation with style must have blinded him to the persistence of her moral purpose" of Agnes Grey.
Split into simpler sentences: F.B. Pinion agreed to a large extent that "Agnes Grey" was quite a masterwork. However, Pinion felt that Moore's examination of the piece was a little extreme and his "preoccupation with style must have blinded him to the persistence of her moral purpose" of Agnes Grey.