Shah Rukh Khan with Emraan Hashmi. Picture courtesy SRK's Twitter account
The book, titled 'Kiss of Life', talks about the hardships Emraan and his wife Parveen Shahani faced when their son was diagnosed with cancer at the age of four in 2014.
SRK and Emraan met and posed for images while holding copies of the book.
Sending his love for Ayaan, the 'Dilwale' actor took to his Twitter handle and posted: "May Allah bless you and your family with health and happiness. Good to see u Emraan. My love to Ayaan."
Emraan shared that the meeting was a "fan moment" for him.
"FAN moment! Lots of love. Thank you for all your support SRK," Emraan tweeted, appreciating the gesture of the superstar, whose next movie is 'Fan'.
Earlier, many other Bollywood celebrities including superstar Salman Khan promoted the book on social media.