The 4WD Handbook covers all a driver needs to know about how 4WDs work and how to techniques for driving them. It is extremely comprehensive and detailed.
The Glovebox Guide is a field reference which complements the Handbook – it covers some of the same driving techniques, but to a lower level of detail.
The Handbook has chapters that the Guide doesn’t, for example how to drive new vehicles, towing offroad and accessories.
The Guide includes information useful as a reference in the field such as navigation and tyre repair – this is not included in the Handbook.
In short, the two are complementary.