This book only represents Imaam Azh-Zhabai's opinion, rahimahullah. It has been disputed. Read it for insight, not for instruction. The major sins are spelled out by the Quran and the Sunna and they are far fewer than seventy.
I know one Muslim brother who left all Islamic duties after reading that book! He told me that he concluded that everything is a major sin and he cannot live that way. I tried to persuade him that the book only represents its author's views but I could not convince him.
Islam is easy, yet many Muslims want to make it hard. As if making it hard makes it better! It doesn't.