A caliph killed his brother. Then he spent twenty years buying books.

You know the feeling. There is something you cannot take back and cannot make right, and the person you would have to apologise to is not available to hear it. So you work instead. You build. And some quiet part of you knows the building is not the apology and will never be the apology, and you keep building anyway.

In 813, Abdallah al-Ma'mun was handed his younger brother's severed head in a palace a thousand miles from home. He asked the entire court to leave the room. They waited an hour outside the shut doors. Nobody knows what happened in there, and he never once spoke about it in the twenty years of rule that followed.

What he did instead was send men into enemy territory with chests of gold to buy Greek manuscripts. He paid translators the wages of generals. He put surveyors on a desert plain to measure the size of the earth, and funded the mathematician whose name became the word algorithm, and tied the money to farmland so that it would keep arriving long after he stopped caring or breathing.

Borrowed Light is a narrative biography of the man who paid for the translation movement that carried Greek science into Arabic and, three centuries later, into Latin Europe. It is told as a story, in scenes, with the people in it speaking out loud. It does not tidy him up. The same man who built it reached for prison at the end of his life to make scholars agree with him, and the book puts that on the page beside everything else.

• Meet the elder son who was passed over for a younger brother, and understand why that single fact broke an empire
• Stand inside a year-long siege of the richest city on earth, told by the people who lived in it
• Watch a rationalist discover, too late, that certainty is the one thing he cannot reason his way out of
• Follow one sentence of Aristotle from Athens to Baghdad to Toledo, through the hands of men who never met and would have disliked each other
• Recognise the particular loneliness of building something good out of a debt you can never repay

This is a book for anyone carrying something they cannot put down and cannot fix, who has ever wondered whether the work still counts.

He never got to say sorry. He only got to keep the lamp lit.