Qur'an. As my Arabic is poor, I'd pick the Tafsir al-Jalalayn in Indonesian. The English translation often miss out most of the good stuff. Religious books are a bit cliche, but here's my reasoning.

1. Information density

The reverse Blinkist effect - summaries of the book end up longer than the book itself. You read the book once, find it nice. Then you read someone else's understanding of it. Compare notes, and find a lot of depth that you overlooked. It's the kind of book which you can discuss one verse over a whole hour.

A lot of the scholars often have a favorite verse or chapter. Mine would be the first 3 verses of Al-Humazah: Don't speak with the intent of harming people directly. Don't speak with the intent of harming people indirectly. Don't count wealth with the purpose of making yourself immortal.

These are very difficult things to do in this era of social media. But it's compact - it fits into a whole English sentence, and makes for a good mantra. When I slip (and it's tempting to insult strangers on the internet!) I can use these verses to remind myself.

2. Motivation/Inspiration/Self-Control

The ideal book should elevate you, give you more control over yourself and your surroundings, and turn you into a better person. Some draw inspiration from religion, but why not just pick a religious book. I love Robert Greene's books because of how clever and practical they are, but they can be a little toxic to practice.

Core to the Quran is trust. Trust that things are working as planned. That doesn't mean doing nothing, you have to be patient and persevere. There's some incredible but logical feats of perseverance, such as refraining from saying hurtful words. There's stories of people who made their riches, and then go out and donate nearly all of it to the poor. And beggars. It requires a lot of willpower to suffer for your wealth and then just give it to someone apparently lazy and incompetent.

Meditation may train your mindfulness muscle, but there's virtues like kindness and selflessness that need to be trained too.

3. Psychology/Philosophy, society modeling

I love a lot of philosophical books but they feel a little incomplete. You learn from it. But can you really apply it and get better at it? What of it?

I sat for a whole night pondering the Quran's An-Nazi'at 79:18-24. This is something anyone speed reading (or reciting) will gloss over, and it's not apparent in most English translations too. Instead of asking Pharaoh to repent, Moses asks whether he's looking for a mentor to purify/cleanse himself. The teacher comes to the ready student, but the student must first open their heart. Then Pharaoh calls his retinue of yes-men, which symbolises that he refuses to be ready. It was at a time when I was stuck on something, and could not find a suitable mentor, and then realized that I was just being stubborn.

4. Entertainment

There's what I call active entertainment. Like you don't just watch a Marvel film, you end up searching/documenting/modeling the world in some way. Passive entertainment is okay, but shallow.

The part I enjoy about reading the Qur'an is that it's all interlinked and ties back into reality. I know if I spend time on it, I find something, and so there's a lot of entertainment value in spending lots of time digging into it slowly. For the last 6 months, I've been analyzing the second chapter of the Qur'an (al-Baqarah) and my notes for that one chapter are about 3000 words long. I have 500 words of notes on An-Nazi'at and it's only 46 verses long. The more I work on one part, the more meaning I can find when working on another part.

By comparison, my notes on a book like Deep Work is about 3750 words. Aristotle's Poetics is the only other book I combed through sentence by sentence, and it's under 2000 words.

Considering this, I think it's a good pick if I really just wanted one book.