6. My book isn't very interesting. My book wasn't

Для выполнения задания необходимо изменить предложение, используя слова yesterday, the day before yesterday, last night, last month, two days ago, last Monday.

В данном случае используем слово yesterday.

6. My book isn't very interesting. My book wasn't very interesting yesterday.

Ответ: My book wasn't very interesting yesterday.