Does it need to be a 'secret' book? If the instruction comes from the Bible, they shouldn't worry that the rank and file might read it.
The reason they want it kept secret is that someone who has access to it prior to meeting with a judicial committee would pretty quickly learn the "tricks" needed to not be DF'ed.
One of the sections is all about "how to tell if repentance is genuine" or some such. There is a series of 15 or 20 questions that elders are supposed to use to determine how "genuine" is the repentance expressed by the "sinner".
Basically, the key is more or less wailing and gnashing your teeth over how you have "damaged your relationship with Jehovah". Embarrassment, shame, bad consequences, etc. all have to take a back seat to how absolutely gut-wrenchingly broken up you are about "bringing reproach on Jehovah's name".
If you put on a good show about how soul-achingly devastated you are about the "pain you have caused Jehovah", there's a better than 90% chance you'll get off with mere "reproof" and not be DF'ed.
If everyone knows in advance what the key to be judged as "genuinely repentant" is, everyone would do it and they wouldn't be able to DF anyone.