When translating The Theological Origins of Modernity written by American scholar Michael Mien Gillespie, he says he thought about giving up twice.

"But, when that happens, all I do is to stand up, take a break, calm myself and then go back," says Zhang.

For Gillespie's book, Zhang traveled to other cities but carried the work along - just to change the environment he was working in.

After sitting up for two months in an office in Beijing without air conditioning in the hot summer, he finished the book, which was rated 9.3 out of 10 by readers on Douban, a popular review website.

The readers described it as "smooth" and not like many other translations of books.

Years of devotion have earned Zhang a good name in the publishing circle, and he is given the freedom to pick which book should be translated.

Zhang has independently planned and translated two series of books.

While Zhang is prolific, he takes his time and does his work carefully.

As the original languages in which the classic books are written are German, Dutch or even Latin, his knowledge of German, which he learned at university, and Latin, which he learned on his own, are of great help.

For some important books, he checks the content in the original language, and compares it carefully with the English version, and adds footnotes.

Commenting on his work, Li Tingting, one of Zhang's editors, says: "He honors simplicity. In his translation, you can hardly find any redundant words."

According to Li, Zhang has a clear idea about what he wants to do and then focuses on it.

While Li says that it takes great determination to devote so many years to translation - which is lowly paid and not regarded as an academic achievement - Zhang says he is lucky to have found a mission he wants to accomplish.

He insists that both academic work and translation are "serious fun" for him.

Zhang says he does not want to publish papers for utilitarian reasons, and he believes that doing translation is more important than writing books at present.

Looking ahead, he says: "There are too many good books waiting to be translated. I have accumulated some experience in translating. It would be a pity not to take advantage of it."