You'll likely have to order the books online as they are a specialty nook. Try Amazon.
I think that when folks here talk about asking simple questions they mean to just take your natural WTF response to a particular subject and phrase it innocently and politely.
For example, when birthdays come up. I know, it's ridiculous. But, you could ask them who they don't celebrate birthdays. You know the response. But, then ask, "So no birthdays or anniversaries or anything?"
Then he'll explain that anniversaries are okay. ( I know, WTF, right?)
So innocently ask, "But aren't birthdays just the anniversary of your birth?"
Then, drop it. He will go on to try to wiggle out of that one, but in his mind will be a little seed of doubt that maybe the birthday ban really doesn't make sense at all.
But, get this, he won't admit that to you. He will think he has given you a fine witness. But, you will know that you have given him something to think about, and it will likely happen subconsciously over time. You are planting seeds that there are logical inconsistencies in his faith. He will hold these inconcistencies in tension with his professed beliefs of his faith and it will eventually get very uncomfortable for him. This discomfort is called cognitive dissonance. When it gets uncomfortable enough from you subtly pointing out things that don't make sense, he may consider the possibility that his belief system is incorrect.
That's just a small primer on the basics. I hope it helps. In essence, you show little bits at a time, but the goal is that he will put the pieces together on his own over time.
The danger is that cognitive dissonance between what you profess to believe and what you know to be logical can also be resolved by digging deeper and more entrenched in your beliefs and ignoring facts, which in this case is you, when folks are pushed too hard. So, don't turn on the faucet full blast. Be the drip!