Complex sentence: Kershaw wrote that few people could really claim to "know" Hitler, who was "a very private, even secretive individual", able to deceive "even hardened critics" as to his true beliefs.
Split into simpler sentences: Kershaw wrote that few people could really claim to "know" Hitler, who was "a very private, even secretive individual". According to Kershaw, Hitler was "a very private, even secretive individual", able to deceive "even hardened critics" as to his true beliefs.