Well, there are two issues here, his representation of Ms. Arias and writing a book about it.
I think if any reasonable person had to pick which one would be more likely to end his career, they would pick the book.
But Nurmi says it's just the fact of having represented her, that may 'prevent him from ever having a real practice'
Exactly what he means is unclear, but he's certainly not referencing the book as a cause for ending his career, he's referencing representing Ms. Arias as 'possibly preventing him from ever having a real practice'. Whatever the heck that means.