Regarding grammar books. I can recommend one small book that will answer most of your questions: The Plain English Guide by Martin Cutts, about $19. Also:
- Writing, Researching, Communicating by K Windschuttle, E Elliott
- The Elements of Style by W Strunk, E B White
- Style Manual for Authors, Editors and Printers put out by the Australian Government Publishing Service, Canberra
On the internet just type in "grammar" and you'll be inundated with sources of free information. But the Martin Cutts book is excellent. My brother bought 100 copies and gives one to every new employee in his PR company and sends them to job applicants who write him grammatically incorrect letters (serves 'em right!).
Let me say a word about how to use books like these, because owning them is not the same as getting good. Do not sit down and read a style guide cover to cover like a novel; you will be asleep by page ten. Keep it by your desk and dip into it. Each time you are unsure about a comma, a hyphen or a tricky plural, look it up, fix it, and the rule will start to stick. Over a year of small lookups you will absorb more than any single reading could teach you.
A warning about the internet. Yes, you can type in "grammar" and drown in free advice, but much of it is American, and some of it is simply wrong, written by people with no more authority than you. American sites will tell you to drop the "u" from colour and to spell "realise" with a "z". For Australian writing, anchor yourself to an Australian authority: the Macquarie Dictionary, the government Style Manual, and the house style of the paper you write for. Use the web for a quick reminder, but let the trusted book settle any argument.
And take a lesson from my brother. The way you write your letters, your emails and your pitches is the first sample of your work an editor ever sees. Get the grammar right there, before you have written a single word of the actual story, and you are already halfway to the commission.