Augusta Huiell Seaman

Three Sides of Paradise Green Augusta Huiell Seaman

Originally published in 1918.
Three Sides of Paradise Green is another Augusta Huiell Seaman book, published a few years after The Boarded-up House, and with kind of a similar setup: Two girls, best friends their entire lives, and a house next door with some kind of history-related secret. Except it's kind of better.
This time the girls are named Susan and Carol, and they've just been asked by their English teacher to keep journals for the coming year. The book mostly consists of Susan's journal entries, although there are a few third-person-narrated scenes.
-Redeeming Qualities Wordpress
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CONTENTS OF "THREE SIDES OF PARADISE GREEN"
CHAPTER I THE JOURNAL IS BEGUN
CHAPTER II NEW DEVELOPMENTS
CHAPTER III THE IMP HAS THE BEST OF IT
CHAPTER IV THE MYSTERIOUS "MONSIEUR"
CHAPTER V TWO ACCIDENTS AND A MYSTERY
CHAPTER VI IN MONSIEUR'S ROOM
CHAPTER VII THE IMP MAKES A DISCOVERY
CHAPTER VIII THE PORTRAIT OF MYSTERY
CHAPTER IX CAROL MAKES A DISCOVERY OF HER OWN
CHAPTER X JOTTINGS FROM THE JOURNAL
CHAPTER XI LOUIS SPRINGS A SURPRISE-AND THE CONSEQUENCES THEREOF
CHAPTER XII WHAT THE IMP KNEW
CHAPTER XIII SUSPICIONS
CHAPTER XIV A SOLEMN CONCLAVE-AND WHAT CAME OF IT
CHAPTER XV MONSIEUR'S STORY
CHAPTER XVI AUGUST FOURTH, 1914
CHAPTER XVII THE IMP MAKES A LAST DISCOVERY
CHAPTER XVIII THE END OF THE JOURNAL