Is there an agent for every novel?

There's certainly not an agent out there for all the sucky novels...

But keep in mind that the correct course of action for an unsellable book is to write a better book

Actually, there are agents who will take on anything. So long as the author pays them enough. Not that any sensible, well-informed writer would want one of these agents, but still. It's worth noting.

And yes, writers should call it quits if their book receives nothing but rejections. But only for that one book: as Kitty said, the best thing to do in that situation is write a better book.

An agent makes his or her living by selling manuscripts to editors. If they don't know anyone who would buy this book, they will not make an offer of representation. If your well-written book is too far off the radar of all the editors currently in the business, your chances of getting a non-scam agent are vanishingly small. Simple as that.

I say call it quits when the submission process becomes too burdensome/upsetting to continue. If it's not fun or life affirming, if you're becoming depressed or submitting is getting in the way of your life--it's time to take a break or stop.

I actually just read this today:

http://www.critique.org/blog/?l=20110413075313

It talks about business models and why some good books don't get picked up.

If you happened to have a vampire romance ready to sub just as Twilight picked up speed, you'd probably be able to sell it. If you wrote that same book today, I'm not sure who would buy it. Likewise, YA today is defined very strictly by age, but a lot of the YA I read had protagonists of varying age (The MC of The Dark Lord of Derkholm is a middle-aged man; Amazon lists it as YA). I don't think an agent would buy that book today simply because of the age, but it's one of my favorites.

Eric James Stone, a nebula-nominated short story writer, is releasing his novel, Unforgettable on his website (http://www.ericjamesstone.com/blog/2011/04/05/unforgettable-cover-teaser/) one chapter at a time (though you can buy it for Kindle anytime). It's fantastic. I wish I could walk into my local bookstore and pick up a copy -- I'm a hard copy person. I don't know if he tried to get an agent for this book, though I assume he did. Maybe it's not what an agent thought would sell -- maybe there's nothing quite like it on the market. I heard a few weeks ago that Scholastic is looking for more MG with Greek gods...maybe something in an entirely different direction might fall of their radar.

Publishing houses publish a lot of great books, but I don't think they catch every good one. I've also heard that agents are more narrow in the scope of what they want to represent than what publishers are interested in looking at, so if you don't have any luck with agents, still sub to publishers (I try not to jump on every rumor, but I may actually believe this one. I've seen a number of agents say they don't accept epic fantasy because it "doesn't sell," and meanwhile PYR just opened up for unagented epic fantasy subs because they don't get enough of it).

My gut feeling is that agents tend to be conservative, because they have to be sure of having several editors that might be interested in your book. Which is unfortunate if you have a book that would be perfect for just one editor but not for anyone else.

I am left wondering if anyone would have picked up my book if I'd gone the conventional agent->publisher route rather than getting a publisher first and then an agent to negotiate the deal. I guess I'll never know...