Reddit mentions of Winning Power: Canadian Campaigning in the Twenty-First Century

Yes, conditions are very good for the success of a third party nominee. The Libertarian Party will do well this year. But no, conditions aren't perfect.

The thing is, Hilary Clinton is fundamentally a normal candidate. She's out of step with her party and is an insider in a year of outsiders, to say nothing of the email scandal. However, America has been run by insiders before; more centrist voters may think that she does not a bold reformer, or that she'll be involved in some scandals during her term, but the business of the United States will function as usual. It's important to remember that during a general election, the candidate closest to the centre of the political spectrum--the least far away from espousing the concerns of the majority of Americans--usually wins. Americans may not like her, but I suspect they'll vote for her.

What would have really changed the dynamic is if both parties fielded outsider candidates, not just the Republican party. Sanders might not be the ideal example here, because he's well-liked, but even he planned to drastically change the economics of the United States, which by its very nature is a new risk to the country and could in theory result in unintended economic consequences and new hardships. The partisans who decide primaries want bold reformers, but general election voters are relatively risk-averse and more likely to want someone who will keep the country running (maintain steady economic growth, respond to unplanned world events, defend against attacks, etc.).

Gary Johnson isn't bad in this regard--he's relatively moderate for a Libertarian candidate, and has a record and the experience of governing New Mexico. However the ideal third-party candidate would probably be a moderate, business-friendly centre-right candidate, who could distance himself from Donald Drumpf and earn the votes of both mainstream Americans and disaffected conservatives.

Regardless of what you think of his politics, Flanagan's Winning Power is an essential read.