Mindy Kaling insists she isn't writing a ''sexy'' book with ex-boyfriend B.J. Novak.

The 35-year-old actress, who split from her former 'The Office' co-star almost 10 years ago, admits they are working on a ''super cool and funny'' project together, but says it isn't a tell-all about their relationship.

Alongside a photograph of a page from the children's book 'Frog and Toad,' the 'Mindy Project' star wrote on Instagram: ''Many of you asked about a book that my friend BJ and I are working on. I heard somewhere it was a tell-all which I actually found exciting, because that sounds sexy and I grew up looking like a little fat boy. But it's not a tell-all. What I CAN tell you it will be super cool and funny and even better than that. For now we are researching. I swear I'm not trying to be mysterious! We are just starting work on it. Here is a cool pic of us working at my house.''

Mindy, who released her first book in 2011 and is set to debut her second in September, recently admitted she and B.J. have a very ''weird'' relationship.

She said: ''I will freely admit: My relationship with B.J. Novak is weird as hell. He is not my boyfriend, but he is not my best friend. I guess you could describe our relationship as a 'romantically charged camaraderie with loud arguments,' but I don't think Facebook would accept this as a new status.''