I am a writer and pay for printing services ($8/book). I sell my books for $15 incl. GST. Can I claim this as regular income and expenses or is it COGS?

As per CRA, you will enter it as income on Line 10400.

If your royalties are from a work or invention of yours and there are no associated expenses, report the income on line 10400.

If there were associated expenses, report the income on line 13500.

Report all other royalties on line 12100.

Completing your tax return

Report these amounts on line 10400 of your return if you received them for a work or invention of yours. Report other royalties (other than those included on line number 13500 of your return) on line 12100 of your return.