Complex sentence: Art asks after Anja's diaries, which Vladek tells him were her account of her Holocaust experiences and the only record of what happened to her after her separation from Vladek at Auschwitz.
Split into simpler sentences: Art asks after Anja's diaries, which Vladek tells him were her account of her Holocaust experiences. They are the only way to find out what happened to her after her separation from Vladek at Auschwitz.