This sentence is found in the author's introduction to a Japanese listening study book.
本書「中~上級編」は、こうした学習者の会話能力の底上げのために日本語教室で5年以上にわたり、1000人以上の学習者に使われてきたものを修正し改良を加えたものです。
My translation: "To help raise the conversational skills of these students, this intermediate~advanced edition is the result of correcting and improving more than 1000 students' _________ over more than 5 years in classrooms."
Is もの supposed to be referring to the classroom teaching materials? The homework? I'm not clear what it's referring to or how it became the origin of this book.