I'm of the opinion, based on everything I've read about Comey, that he did not do this intentionally to harm the Clinton campaign. I think he really was trying to cover his ass. I also think he displayed colossally poor judgment. I understand his reasoning, but it's still faulty.
The most likely inference based on what we know is that these are work-related e-mails sent from Clinton, and probably others in the state department, to Huma Abedin, or from Abedin to Clinton. Which makes it highly likely that most if not all of them are duplicative of what the FBI has already received. Comey should have done a preliminary review to at least determine the degree of duplication with what they've already seen before going public with this shortly before an election. I doubt this takes much time, and could probably be aided by software. If it turns out that there are very few e-mails, or even none, that are new to the FBI, then a complete analysis can be done in no time. If there are lots of new e-mails, thousands or tens of thousands, THEN maybe consider telling the public that they have found a lot of new e-mails and that it will take time to review them.
So far as I'm concerned, Comey has created a mess by putting this letter out which at this point is nothing more than an innuendo that can nevertheless have a large impact on the election because most voters have no idea how to interpret it. He had no business doing that without knowing the first thing about these e-mails. Now he needs to clean this up, and frankly, I don't give a damn if his staff gets no sleep for the next week. The Clinton campaign's calls for detailed information about these e-mails are appropriate. I'm not yet ready to call for Comey's resignation, but if he is just going to drop this bomb and let it hang out there until after the election, then he definitely needs to go.