There are international tax agreements. Some countries require simply filling out forms, some require you to notarize the forms, others require you to get an international tax number. But you only pay taxes in your home country. This is another reason why agents are the best. They know all this stuff and talk you through all this stuff
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As to the question at hand: get whichever agent would best represent your work in the best market for your book. Remember too that while Canada and the States are closer by proximity than the UK, there's really no difference between subbing to agents in those countries - they are still separate countries from Canada and each one will claim Canadian publishing rights likely (when I was agented in the UK and sold to the UK the publisher wanted Canadian rights, and then when I was agented in the States and sold in the States the publisher wanted Canadian rights. Either one, they tend to get Canadian rights, though sometimes your agent will hold those back and sell them separately. Usually though, we're part of the deal
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