Due to my business I am self employed and I fill in a tax return each year. I assume that any income I might get from book sales is taxable. What is deemed tax deductable? For instance are the services I pay for with Writers and Artists, printing costs, postage etc.
Yes.
I'd have thought all those you mention are. You can claim for holidays if you can prove they're essential research tools, though I've never tried that.
If your main income is from writing (or other artistic endeavour) to can offset your tax liability if, for example, you get a large advance, which would normally be taxable that year.
I wish! *sighs*