The series has a lot of appeal for many readers and I think that is why it's so iconic. 1) It is a ongoing immense series that has not slowed momentum. This is attractive to readers who can't get enough of one particular author or series. Janet Evanovich readers know that there will always be a book in the works, and that their wait is short. 2)The ever evolving and changing cast of characters and awkward obstacles bring light hearted comedic elements to this series that will have you dying with laughter. 3) This series has elements of erotic romances that give it a rom-com feel. There are several handsome romantic interests that support the plot movement, and continuously show up through out the series for fan service. 4) The contemporary setting in New Jersey is unexpected and brilliant. It gives you those nostalgic mob moments.
There are many more reasons to pick this series up, but those are the most prevalent in my mind.