Dictionary / Vocabulary / 口語体
こうごたい
colloquial style; written style based on spoken language
その引用した文章からわかると思うけど、口語体で書かれた本です。
I expect you can tell from the quoted text, but this is a book written in a colloquial style.
これらの話は口語体の日常の言葉で語られる。
These stories are told in colloquial and everyday language.