Yesterday as a result of a new story, I discovered that the Navy style guide is now online. If you write about SEALs or other sailors, this might be useful for dialogue and official jargon.
Here are some other links that might be useful if you're investigating questions of style.
AP Stylebook Online
AP advocates a style which is less formal than academic writing but not as casual as dialect. If you write for newspapers, magazines, or genre fiction publishers, you'll generally be safe with AP.
The Economist Style Guide
This concise guide is useful if you want a more thoughtful tone in your prose. Is your character highly educated? Is he a CEO or other money guy? This might come in handy.
The big three of academic style. Most of the ordinary manuscript format rules (double-spaced, one-inch margins, page numbers in corners, etc.) come from academic style.
Strunk & White
A general style guide aimed at ordinary writing. This is really basic stuff. No excuse for not knowing these rules. (I have special love for rule II.7, of course.)
Then there are the specialized style guides and online dictionaries. Is your character a doctor? Maybe you need to look at AMA style for that memo he writes in chapter six. Was he born and raised in London? Maybe you need this English slang dictionary. Is your hero a retired athlete? Here's a brief sports terms dictionary.
The point is that there are multiple layers to manuscript style. It needs an appropriate format to be ready for submission. It needs appropriate grammar and mechanics and usage within the text itself. And it needs characters who sound like the people the author intends them to be. Style guides can help the writer achieve all of these goals.