I'm assuming that your Sim is Level 10 in Writing? If they are, there is a 10% chance any book is a Best Seller. When a Sim "receives an advice" from the "Journalism Boss", 5% is added on to the chance of writing a Best Seller. There is also an opportunity that adds 10% to the chance of a Best Seller (I don't know the exact name, sorry). Every book you write in a genre will add 1.5% to the multiplier, to a maximum of 60%. Also, a <Genre> Novelist will have 20% added to the chance. If you want to guarantee a best seller, I recommend that you do all the above, except for the thing about the Journalism Boss. All in all, if you do it all, there's a 105% chance.