I was going to suggest 'The Complete Plain Words' by Sir Ernest Gowers as an old fashioned (i.e. printed) possibility - my copy dates from 1963, priced 3/6.
When I looked to see if it was still in print (a revised version is) I found it as a free download - not quite an online guide but getting closer... http://www.scribd.com/doc/217456602/Complete-Plain-Words-Sir-Ernest-Gowers-pdf
I would rep you if it were still a forum function.
I'll have to dig out my old copy, recommended at secondary school for General Studies essays back in the '70s.
Meanwhile the pdf you found will be loaded onto sons laptop.
@mespilus started
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I was going to suggest 'The Complete Plain Words' by Sir Ernest Gowers as an old fashioned (i.e. printed) possibility - my copy dates from 1963, priced 3/6.
When I looked to see if it was still in print (a revised version is) I found it as a free download - not quite an online guide but getting closer...
http://www.scribd.com/doc/217456602/Complete-Plain-Words-Sir-Ernest-Gowers-pdf