Complex sentence: He testified that Irving had written a "very, very effective piece of historical scholarship" in the 1960s, which was unrelated to his controversial work; he also said that Irving was "not in the top class" of military historians.
Split into simpler sentences: He testified that Irving had written a "very, very effective piece of historical scholarship" in the 1960s, which was unrelated to his controversial work. He also said that Irving was "not in the top class" of military historians.