I am pretty certain that the same text to which you refer above (the Lynne Truss, not Hart!) will agree with me.
Lynne Truss does indeed allow the apostrophe to indicate plurals of words, as in do's and don't's, but's and and's. Well, never trust a book that doesn't have an index, that's all I can say.
Personally I'd put more faith in The Oxford Guide to Style
which, as Paul points out, stipulates that, where confusion might arise
, you should either put the word within quotes ('do's and 'don't's) or in italics (do
s and don't
s - note the s
is not italicised). Otherwise it advises against using the apostrophe when creating plurals, including abbreviations such as QCs, SOSs, the 1990s, etc.
I'd hate anyone to construe from the above that I think I know anything about English. I was once conned into spending a few weeks teaching it to a group of Swedish teenagers. It was one of the most embarrassing experiences of my life: the two adult Swedes in charge of them knew far more about how our language worked than I had ever learnt. Still, some of the girls were fun.