但是书籍是人类最忠实的朋友，是人类进步的阶梯。

However, books are faithful friends of mankind and steps to human progress.

youdao

众所周知，书籍是人类的朋友。

As we all know, books are close friends of humanity.

youdao

书籍是人类最宁静和永恒的朋友，是最善解人意和最具智慧的顾问，是最有耐心的良师。

Books are the quietest and most constant of friends, they are the most accessible and wisest of counselors, and the most patient of teachers.

youdao