What is the book of deeds in Islam?

Who writes it?

The Quran says every person has noble angels assigned to them, who "know whatever you do" (Quran 82:10-12), and that not a word is spoken without a watcher ready to record it (Quran 50:18). Muslims often call these two angels the Kiraman Katibin, the "noble scribes". See Do Muslims believe in angels?

What happens on the Day of Judgement

The next verse says: "Read your record. Sufficient is yourself against you this Day as accountant" (17:14). Each person will read their own book. Those who receive it in the right hand will joyfully say, "Here, read my record!" (Quran 69:19). Those who receive it in the left hand, or from behind their back, will wish they had never been given it (69:25, 84:10). Another verse describes people amazed that it "leaves nothing small or great except that it has enumerated it" (18:49).

How Allah writes the deeds

The record is not cold bookkeeping. The Prophet ﷺ explained that Allah's system is generous (Sahih al-Bukhari 6491):

• If you intend a good deed but don't do it, one full good deed is written for you.
• If you do it, it's written as ten to seven hundred times, or more.
• If you intend a bad deed but hold back, a good deed is written.
• If you do the bad deed, only one bad deed is written.

Children, the insane and people asleep are not held accountable: the pen is lifted from them (Sunan Abi Dawud 4403). And the Prophet ﷺ described Allah privately reminding a believer of their sins, then saying, "I covered them for you in the world, and I forgive them for you today" (Sahih al-Bukhari 2441).

Can I change what's in my book?

Yes, while you're alive. Sincere repentance wipes out sins, and good deeds remove bad ones. Converts start with a clean page at their shahada. See Are all my sins forgiven when I become Muslim? Some deeds even keep adding to your book after death, like ongoing charity and beneficial knowledge. See What is sadaqah jariyah?