Abbe,
Without meaning to sound trite, but experience is much more important than a book. The basics of taking bearings, backbearings, setting and reading a map, and reading terrain are straightforward enough. Knowing how that applies to travelling in the wilds is only gained by lots of application.
It's always instructive to prepare a route card for any walk and then compare this to what happened on the actual walk. Were your time estimates correct? When you paced it out, how (in)accurate were you? When you got to the end of each leg, how far from the intended point were you? To what extent did the ground underfoot, terrain, and weather impact your accuracy? (It's difficult to do this if you're leading a group, as your mind won't be fully on it: worthwhile doing a few dedicated sessions!)
Any map and compass navigation will have a certain amount of innaccuracy built in. Terrain and weather will increase both the innacuracies and your ability to recover from them. It's only by experience that you begin to appreciate these impacts and learn how to allow for them.
In any case, where these four were concerned, you would have to question why they went out at all given the weather forecast! Watched "touching the Void" last night, which just goes to show how even with experience and the right equipment you can have a lot of problems. You shouldn't mess with extreme weather in extreme environments. I hope you're right: I hope they learned their lesson!
Slan,
Mal