Thanks. I actually do have "Sisters...", but I haven't read the entire book yet... I was basically loosely going by one of Nicholas's diary entries where he wrote that Marie and Anastasia burned their diaries... He specifically mentioned the two girls. I wonder what made Marie keep those 3 notebooks and burn the rest.
Possibly she was only able to burn her current diary. That is, there's a good chance that her three earlier ones simply weren't at hand, as in storage perhaps. There wasn't really time oropportunity to organise a proper bonfire, which might have been pretty big, or suspicious. Or, equally possible, she may have felt, rightly, that the earlier diaries would be seen as less 'incriminating' for her parents, at least as opposed to a 1917 one which might have included things about the war, political unrest, etc.