I like a lot of what Truss has to say but I find that sometimes she relies on terminology that looks clear but is not, on reflection, self explanatory. With respect to semi-colons and colons, I think it's possible to give more precise guidance.
Semi-colons are used between sentences where the writer wishes them to be treated as a list. They separate sentences as commas do items in lists of noun phrases and so on. The point is that although we are invited to interpret each sentence in relation to the others they all bear more or less equal weight. For example,
Summer ended; flowers faded; leaves turned from green to gold, red and brown.
Colons are used between one sentence and another sentence or a sentence fragment where we are specifically invited to interpret the latter in the light of the former. So, we expect the second element to throw some specific light on the interpretaion of the first. For example,
He divorced her: he no longer loved her.
The relation is of reason-result: the second element provides the reason for the action described in the first.
If we look at the semi-colon example I gave again we can imagine another sentence:
Summer ended: flowers faded.
This time we are specifically to interpret "flowers faded" in the light of the sentence "summer ended". The relation between them is one of statement-exemplification or arguable assertion-evidence.
As it happens, I'm the opposite of a prescriptivist when it comes to grammar and spelling but I think that understanding both as well as you can is empowering. Knowing what the effect of using a colon is gives the writer more fine tuned control in trying to provoke a certain response in the reader.
For what it's worth this analysis of clause-joining is based in part on the work of the distinguished linguist Winnie Crombie and anything worthwhile is to her credit and all mistakes or infelicities are down to me.
Jimbo[/b]