I found the book to be quite a bit more difficult than I had expected.
I found the book to be a harder read than I had originally thought.
1) I found the book to be quite a bit more difficult than I had expected.
「読んでみたら思ってたよりかなり難しかった。」というニュアンスです。
2) I found the book to be a harder read than I had originally thought.
「読んでみたら最初に思ってたより難しかった。」
found を使うことで「読んでみたら」のニュアンスも含まれていますので、英語にしなくても大丈夫ですよ!
ご参考になれば幸いです!