In my contacts with other writers, I am continually amazed at the lack of interest in the mechanics of writing. It's as if Word's spellchecker, appalling grammar checker and an unused dictionary and thesaurus will do – anything else is either irrelevant or simply garnish.
Of course, they will often also have been given Eats, Shoots and Leaves for Xmas (and may have read it) but that's about it. Many will not even know of the existence of other guides that could make their lives simpler, their writing better and arguments with clients easier to win.
The following are the reference guides I use pretty much every day.
Once you've got a book that tells you how to spell, the next thing you need is one to help you put your words together correctly, punctuate them accurately and maintain consistency. Enter the style guide. Some clients will go to the time and expense of creating their own (I've written one myself). However, with a very few exceptions this is a waste of time and money. Instead, simply buy one off the shelf.
By far the best style guide on the planet is The Chicago Manual of Style. It's very easy to use, clear, concise and right 99.9% of the time even though it is aimed at US English writers (double check spellings if you use it for guidance on word usage).
If you want to be a stickler for UK English, there's The Oxford Style Manual but, in my experience, it's a bit old-fashioned and leaves a lot to be desired in the usability stakes.
Beyond this there are a bunch of others: Guardian Style (a bit lightweight though good on confusing words), The Penguin Writer's Manual (again, a little old-fashioned but with a good style section), and Chambers Good Writing Guide (a little basic in some places but a good overall guide). Then there's the Economist Style Guide which, in my humble opinion, should be pulped.
Finally, another American interloper – The Elements of Style by Strunk and White – a cheap, pocket-sized guide that's worth reading just for its chapter, 'An Approach to Style'.
Of course, some things are just technical. Sometimes you need to know 100% what's right in UK English. For that you need The Cambridge Grammar of English. Yes, it's almost a thousand pages long but it's surprisingly usable and will more often than not give you the definitive answer you are looking for.
So, if you're serious about writing (or even not that serious but don't want to look like an idiot in front of a client), pop over to Amazon and buy The Chicago Manual of Style, The Cambridge Grammar of English and The Elements of Style – it'll be the best £60 you'll spend this year.