Reddit mentions of Escaping the Rabbit Hole: How to Debunk Conspiracy Theories Using Facts, Logic, and Respect

You might find Mick West's book, Escaping the Rabbit Hole, to be helpful. It's all about how and why otherwise smart and reasonable people can fall into conspiracist beliefs, and how to approach the topic with them. It also goes into some of the details of the arguments behind specific conspiracy theories like the 9/11 ones. And it's a surprisingly good read, IMO.

Highly recommend Escaping the Rabbit Hole by Mick West which was written with exactly your situation in mind. It is written in a sympathetic way to help 'friends' discuss conspiracy theories in a way that is non-confrontational and should maximise dialogue. He also includes some very thorough debunking of popular tropes about 9/11 (including AE911 Truth and the insurance claims, IIRR). Sections ask you to 'ask your friend...' to help give you the tools to raise issues.

I disagree with other people here that 'truthers' don't listen to evidence. That is categorically wrong and counter-productive. Everyone goes through stages in their life when they believe certain things that turn out to be poorly informed. Although it won't always work, making your friend aware of counter-arguments against his claims is very important. Sure, sometimes they will just reject your claims out of hand, but often they are not aware that there is a counter-argument. It's always worth giving it a go.

Source: Used to believe in plenty of conspiracy theories.