Being a little bit old school, I'll start with a book (now out in hardcover and Kindle editions, shortly in paperback): SEND: Why People Email So Badly and How to Do It Better (Vintage)by David Shipley and Will Schwalbe. It expands "style" to include decision-making around when to send and to whom, as well as email content advice, and it's well-written to boot.
But new-school-types may not want to read a guide. For them, a new tool called ToneCheck ("Just like SpellCheck...but for tone") may be the solution--or at least, a type of seat-belt to keep their emails from crashing into a verbal pole. This Fast Company article ran a few tests of the service, available only for Outlook. From the article:
"It identifies words, phrases, or sentences that exceed your tone tolerance," [ToneCheck CEO] Eldridge says. After downloading the software, you set the tone you want—a scale from negative to positive. When your email is ready, you run the check, and the program spits back flagged phrases and offers alternatives.
Your choice: Analog book or email plug-in. Readers, share your sources for email improvement in the comments!