A LOT of GP guys use Autotap. Check out a state website like ILCGP, MNCGP etc, the big one will be a PITA.
Basically the more knock you have, the more dangerous it is to drive/mod the engine. Many mods are devoted to eliminating knock. Knock robs power, and true knock is caused by detonation in the cylinders. If you're reading a lot of KR, you'll want to use that information, and some trouble shooting to get rid of it. You can also use it to find your limits on mods before an IC, etc.
O2 readings are key to see how Rich or lean your car is running. This is important once you start doing things like a Cam, Rockers, Nitrous, blower upgrade, fuel system upgrades.
Intake temps are important if you're trying to build a CAI. If you're within 10 degrees of ambient that'* a good start usually.
There'* no book on this, you just use it when you want to know exactly what you're doing.
'06 Vee Dub GTI 2 door Tornado Red
- 2.0 Turbocharged, Intercooled, Direct Injected, 6 Speed DSG, Underbody Aero, 17" Classix
- Moonroof, Bi-Xenon headlamps, Interlagos Plaid Sport Seats, 6 Disc In-dash, iPod integration, GTI Monster Mats (winter)
'93 Bonneville SE Purple Pearl H4U package - picked up June 23, 2003, Traded-in June 28, 2007...