I wanted to change textbooks because most of the old one was just reading.
I wanted to use a new book because the old textbook was mostly just reading.
どちらも「古いテキストはほとんどが読むばかりだったから変えたかったんだよ。」という言い方です。
1) I wanted to change textbooks because most of the old one was just reading.
most of the ~ で「ほとんどの〜」という言い方です。
2) I wanted to use a new book because the old textbook was mostly just reading.
mostly 「大部分は」を使っても言うことができます。
mostly just reading で「大部分はただ読むだけ」
ご参考になれば幸いです!