I wish to know what sort of books he has mainly written in Japanese;I am answering the question here, as it might be of interest for other readers, too.
are they serious academic treatises or popular children's books to encourage students about science?
He has published three books so far in Japanese. He also contributed to another book which is a collection of articles written by many people. However, here I focus on the three books, of which he is the sole author. They are not academic publications, but for general public. The books are thought to encourage students about science, although they are also read by adults. Even when he talks about "science", often the actual content is more like a fantasy.
1. "Uchuu Elebehtah" (Space Elevator) published 2006/6/22
2. "Taimu Mashin" (Time Machine) published 2006/11/23
3. "Poketto no Naka no Uchuu" (The Space in Your Pocket) published 2009/8
Although the title of the first book is "Space Elevator", there is not so much detailed discussion of space elevator (although there is some explanation, of course.) The rest of the book is mixture of his personal experiences and his essay on various scientific subjects.
In the second book "Time Machine", he tells about his experience of trying to make a time machine when he was 15 years old. (The story already appeared in "Space Elevator" but he writes more details in this book.)
The third book "The Space in Your Pocket" is again mixture of his personal experiences (a kind of autobiography) and essay on various subjects (scientific and otherwise). Actually there is some overlap in his stories, with the other two books (it is not just a copy from the other books, but he tells again about the same story in several places.)
I do not know precise sales information of these books, but it seems that they have been selling well. All the publishers of his books are well-known, respected publishing companies in Japan.
Many of his personal "experiences" and author information in these books are now questioned. For example, see the posting on the "astronaut candidate" claim. We are also curious how his stories told in Japanese books are compared to what is told in Turkey. I will discuss these points later.