"I have a question regarding non-US agents. I've written a suspense/thriller novel, which earned honorable mention in a fairly prestigious UK competition. The novel is set in Wales, and I received a manuscript request from a London based agent.
I'm in the US, and plan on submitting queries to US agents. Is it normal, or even proper, to have different agents for US or International publication?"
As a rule of thumb, it is best to focus your search on finding an agent in the U.S., for several reasons. First and foremost, if you submit your book to foreign publishers, whether directly or via an international agent, the first question they will ask is who is publishing it in the U.S. When you tell them there is no U.S. publisher, then in most cases they will either lose interest, or tell you to come back to them once you have a U.S. publisher, or make you an offer which is lower than it may have been otherwise.
There are other reasons to find a U.S. agent first. When a U.S. agent shops your book to U.S. publishers, he needs every option at his disposal in order to make a sale. Landing a book deal is not easy, even for an agent, and if an agent is forced to shop around a book in which world rights are not available, that could end up making the difference in his being able to place it at all. It may end up that a U.S. publisher likes your book, but is somewhat on the fence, and having the assurance of world rights makes the difference, and enables them to make an offer. Alternately, the advance that a U.S. publisher offers might be significantly smaller if world rights are not included as part of the deal. For example, a publisher might offer you an advance of $50,000 for U.S. rights only, or $75,000 for world rights. If you have already engaged one or more international agents to shop your book in their territories, then you will not be able to offer those rights to a U.S. publisher. U.S. publishers like to engage their own international co-agents, and will not want to use yours. That is not to say that in every instance you will give a US publisher world rights—but you do want to all have options at your disposal.
Additionally, once a U.S. agent sells your book to a U.S. publisher, if he retains the world rights, the first thing he will do is put your book into the hands of all of the international co-agents he has relationships with. If you have already committed your book to other international agents, it will cause a problem with your U.S. agent.
All of this to say that you should not query international agents as the first step towards getting published. I still recommend your approaching U.S. agents first, and, of course, that your U.S. agent approaches U.S. publishers first.
If your U.S. agent shops your book to U.S. publishers and it doesn't sell, then, at that point, you might want to consider shopping it in other countries. In that case, your U.S. agent may be willing to engage one or more of his co-agents in other countries to try to shop them. That, though, would be fairly unusual. As I said, making foreign deals without a U.S. deal already in place is not an easy sell, because the first question asked will be who is publishing in the U.S. That said, there have been some rare cases where a book did not sell in the U.S., but then landed some foreign sales, and then the agent came back to the U.S. with the momentum and made a deal here. That, though, is very unusual.
Of course, if your novel is set in a particular location overseas, for example London, then I could understand how you might want to query UK agents directly, and I could see the temptation to have it shopped in the UK first. Still, though, I would hold off and wait to see what happens in the US first. In your case, if you are unable to find a US agent, and have exhausted all possible submissions, then, in that case, you have nothing to lose by following up and submitting your manuscript to the UK agent who has already contacted you directly.
Along these lines, for you authors who are self published and have had foreign publishers approach you directly, possibly as a result of your sales on Kindle, even in that case, I wouldn't necessarily recommend entering into a book deal with foreign publishers directly. If a publisher in a particular country approaches you with interest in your book, then chances are that there are other publishers in that same country who may also be interested in your book. Your book should be shopped thoroughly in that country, as opposed to taking the first offer that comes your way. Additionally, and more importantly, it would be better for you to use the international interest in your book as a selling point to help convince US publishers to publish it in the US. As I said, it is not easy to land a book deal, and if you can prove to a US publisher that several international publishers are already interested, then that might make a difference.