For an adult who always assumed she was clever enough, i have a degree, but my spelling and grammar have alot to be accounted for.

Please give author names or book names so i can look into my local bookshop.

Many thanks

dictionary

By Collins or Oxford.

the dictionary

Try Eats, Shoots and Leaves, I can't remember the author's name but I am pretty sure it's stilll available on Amazon.

For grammar you absolutely must get the Elements of Style by William Strunk. It was originally written in 1918, then E.B. White took it over in the late fifties. I think the most recent edition is 2005. It is wonderful to read, and fun too. You will be amazed at the number of good and easy-to-understand tips. I bought an illustrated version for my daughter a few years ago, which was beautiful as well as practical.

For spelling, nothing beats the regular old dictionary. For the web I like dictionary.com.

I'd recommend "Eats, Shoots and Leaves" for basic grammar; it's a very accessible text.

However, over all your best strategy is to read more. You'll find that the more you read, and the better quality books you choose, the greater your abilities in SPAG will become. Not to mention your vocabulary will improve.

I agree with Tictocsailor. The Elements of Style by William Strunk & EB White is probably the best book out there for grammar if you had to pick just one. It's extremely easy to understand and it's quite small too, so you won't have to wade through tons of pages to find what you need. For spelling, a dictionary is your only choice as far as I know.

The best work is Fowler's Modern English Usage. (proper titlte is : Burchfield, Robert William (1996). The New Fowler's Modern English Usage, 3rd edition, Oxford University Press.)
Or Bill Bryson's Dictionary: For Writers and Editors - quite readable and good for explaining subtle differences between words and how they are used.

Also good is Sir Ernest Gower's, 'the complete plain words' which was written to help train the civil service to write clearly and grammatically. Lynne Truss's Eats Shoots and Leaves - rather limited book, but amusing on the subject of apostrophes and hyphens.

You can find these in your local library - see which you think you will get the most from before purchasing.

The dictionary will obviously help.
Try looking into the advanced section for learners of English. The advanced books which help people to learn English come in all forms including advanced grammar and academic writing and the best ways to approach it.
Bearing in mind that they are wirtten for people whose first language is not English it will be far easier for you to use and will focus specifically on grammar.