I read somewhere that an investigation was holding up until his book came out in order to really NAIL him. Not sure the person tweeting this is right but it seems it might have been a Thomas Wictor tweet. I have a lot of respect for his opinions. He seemed to think Comey is cocky enough to actually put in writing incriminating testimony.
Could be, but I think more likely the timing just happened. People can lie in a book, to the point of defmataion. Comey could admit he was involved in a plan to assassinate JFK, and it wouldn't add any risk of being charged.
If the fact ever became a question in court, what controls is his court testimony. The book is pure political theater.