How to get a best selling book?

Have you tried writing when your sim is inspired/very inspired?  

Yes

@Wolfie_GirlXox Just keep on trying. Sometimes it takes more than several times. As long as you keep at it eventually it will work out fine.

Have fun playing. 😉

@Wolfie_GirlXox Did you try writing all sorts of genre? It can help when you write SF, fantasy but also when in a self-assured mood authoring a motivational book can become a bestseller. When flirty write a book of love. When sad, a sad story can help. The more genres you have tried the more chance your Sim will make writing a bestseller. Watch out for writer's bock, though.

Good luck with your Sim's career! 😉

@Wolfie_GirlXox There are some really good suggestions here.  I just thought I would add, from my own experience, I've only had bestsellers when my sim had a bunch of stacked moodlets/buffs.  I think it's best to start building those moodlets/buffs first thing in the morning, after a good night's sleep. Maybe try some of these things to build up those moodlets/buffs.

- Take a thoughtful shower

- Eat an excellent breakfast

- Browse art on a computer

- Research something on a computer

- Read a book

- Go jogging or have a workout

- Woohoo (if your sim has a romantic partner)

- Play an instrument (even if they have low skill)

- Watch the cooking channel on TV

- Listen to the radio and/or dance

- Talk to friends (especially sims for which your sim has sentiments)

This is not a comprehensive list by any means.  Based on your sim's traits and likes, there are many things that will generate moodlets/buffs.  I think if your sim builds up enough of them, you'll have your bestseller.  Hope this helps!

Depending on what packs you have:

Take a thoughtful shower.

Cloudgaze

Use the Home Studio lot trait

Also try children's books with all that.