Writing is a little like presenting in that many people with much to share are great at communicating informal conversations and e-mails, but have a basic fear with either presenting, or with the prospect of "technical writing." With presenting, the way forward is easy - start with a small audience like a user group or a local Toastmasters chapter and build from there. With technical writing the approach is more like learning to code - the best path forward is to get a good desk reference and start reading other people's code to see how concepts are pieced together.
The best desk references for writing are style guides. Style guides aren't something you need to read cover to cover. Typically I flip through, read the interesting bits on language and style, and come back to the guide to look up specific pieces like citing references, labelling diagrams, terminology, and so on.
The place to start, and the best book ever written on writing is Elements of Style.
The Microsoft Manual of Style is a terrific guide to technical writing, though the Kindle version is to be avoided as the formatting did not translate well. The 4th edition brings it up-to-date with guidance on writing about mobile devices, SEO and other modern topics. With a little digging around the web you can turn up digital copies of the 3rd edition, which offers good advice on writing (e.g.: Ch. 3 on Globalization, or Ch. 7 on Tone and Rhetoric), though some parts are now dated (e.g.: the section on COM).
For formal academic papers the guide to follow is the MLA Style Guide, and it looks as though there is a variant specific to "scholarly publishing" now. Also check out your local university bookstore - the U of S used to publish a great booklet that summarised the essentials for citations and formatting, and I see now they offer a nice Editorial Style Guide online with guidance for business writing.
Not directly related, but one of the more enjoyable style guides is the Washington Post Deskbook on Style. This is the book where I first learned the hallmarks of a good obituary and why news agencies always refer to the "alleged" crime. Plus I like the writing style of the Post and found it interesting to read from where that mindset flows.
Reference in hand, the next step is to "start reading other people's code." MSDN is awesome in its consistency and the best example of all their style guide has to offer.
If writing for an academic audience, check out what Microsoft Research (MSR) has to offer. MSR publications are mostly written as formal papers closer to what the MLA guide prescribes, and reading a few will give you a feel for the language and layout. They're also simply great reads on topics that often turn into products in months and years ahead.
Conclusion: If writing a whitepaper or even a book, the Microsoft Manual of Style and The Elements of Style will serve you well. If writing for a more academic audience then get the MLA guide and read a few academic papers to get a feel for the language and layout. If you have a specific publication in mind, then contact their editorial staff as many have their own guides or templates for submissions.
Are they any other great resources you recommend? Sound off in the Comments!