Less than Comparison using na-Adjective
This structure is used to express that A is less than B using a na-adjective.
Examples
この店はあの店ほど静かじゃない。
This shop is not as quiet as that shop.
彼はあの人ほど優しくない。
He is not as kind as that person.
その本はあの本ほど面白くない。
That book is not as interesting as that book.
Quick check
その本___あの本ほど面白くない。