Is it permissible to keep the books of a person who lost my file, knowing that he has not asked for them and I am not comfortable with this action internally?

The obligation to guarantee a borrowed item if it is damaged or lost is a matter of scholarly disagreement. The preponderant view is that the borrower is absolutely obligated to guarantee it, whether he was negligent in its use or preservation or not. This is based on evidence, including the hadith of Safwan ibn Umayya: "Rather, it is a guaranteed loan," and the hadith of Samurah: "The hand is responsible for what it takes until it returns it."

If the file has significant value, you may ask your friend for its value, or both parties can agree to make the books a compensation for it. You are not permitted to keep his books without his consent, due to the hadith: "Return the trust to the one who entrusted you, and do not betray one who betrays you."