Prabhupada - the ISKCON luminary believed in NASA's fake moon landing and secret government. From this book by one of the ISKCONite, It seems he believed in flat earth also owing to literalism.
Did he actually believe in flat earth theory?
From the book which you have linked in your Question, answer depends upon what Bhagvatam says about the shape of the earth (which seems No).
On page 147, the author writes:
My second question to Srila Prabhupäda would be this: why does Srila Prabhupäda use the word bhü-gola to describe the Earth as a round planet? For example in this conversation of 1973:
Prabhupäda: Yes, world is round. That is fact. Goloka. In Vedic literature it is bhü-gola, jagad-anda. These words are there. We can see also it is round, jagad-anda. The universe is round. And Goloka. Or Bhü-gola. Bhügola, the earth is round. So in the Vedic literatures... Therefore their knowledge is also imperfect because they do not refer to the Vedic literatures. It is already there. Bhü-gola. Bhü means the earth; gola means round. It is already there. And the geography's called, according to Sanskrit, it is called Bhü-gola. Long, long ago, before Galileo. So if the state is blind, he does not see whether he's talking right or wrong, then havi candra raja gobi candra mantri (?). What can be done? That is going on. (Morning Walk, Dec 9, 1973, Los Angeles)
Prabhupäda's last statement about this topic in the book is:
Prabhupäda: Take the version of Bhägavatam.
(Discussions about Bhü-maëòala, July 5 1977, Våndävana )
The word Bhü-gola is found in Bhägavatam which is explained in below articles/blogs by other ISKCONites and in related post.
Articles/Blogs by other ISKCONites which discard flat earth theory: