To avoid a break in an agent/author relationship, as a result of a book not being picked up, it's highly advisable that you're busy on your next title the minute your agent starts the shopping process. If you time it right, you'll have that next book written and polished by the time the first book has exhausted all leads. If you're a solid producer, there is no way an agent is likely to drop you. A book usually wears out on the submission trail from nine to twenty months. That's an approximate. In my case, 18 months is about the death toll, but there are agents out there that will champion a book for several years. DO NOT SIT ON ONE REPPED BOOK, WAITING FOR A SALE.
One snag...
To make sure that the agent is going to look favorably on your next book, I usually write the synopsis and first three chapters and send them to him and ask if this is something that they might be enthusiastic about. In other words, it has to ring their bell. I do this as a precautionary measure. My agent will take many things into consideration--if there is a market for it, if it has a good concept/premise--that all important hook, and so on. You don't really have to send the first three, but I would run the idea by them and ask for a yea or nay. Agree to hit on something that gives both of you the happy shivers.
There is nothing more devastating that writing full tilt on something, spending six months, and then having your agent tell you that it isn't going to fly. Your agent has to fall in love with it, too. I had this happen to me. Fortunately it was a trunk novel that I brought out of moth balls and reworked. He still did not like it. That's one that didn't get through the gate.
Hint: Ask your agent what's hot out there. What are they looking for? What stands the best chance of being picked up? If he/she has an idea, and if it falls within your genre parameter, and you've got the voice/style to carry it off, go with it. You don't have to do this--it just ups your odds. I'm not saying to hack it. I'm saying to strategize with your agent. They have their pulse on the trends of the marketplace.
As far as agents dumping clients. I don't see a lot of that happening. What I see more is writers getting frustrated with their agents and dumping them. Case in point: I've been with my agent for 22 months. From the beginning, he had 29 fiction writers in his stable. Today he has 25. The evidence shows that three of those writers left him. He dropped the other client for personal reasons. He also represents a dead author, who started out with him at the very beginning. He made a vow that he would continue to champion that book out of respect to the estate. And, of course, he's wild about the book. Now, THAT'S diehard service going above and beyond the call of agenting. In anyone's book.
You're going to get genre "niched" whether you like it or not. I don't even think that's a word. Your maximum sales potential will come from a fan base that is launched from your first book. If it does even respectively well, you'll be encouraged to stay there, for strickly marketing considerations. It will do you no good to write all over the genre topography and hope that one of those books hit. Your agent will know where your strengths lie and tell you so. You have only to ask. When your last name is King or Koontz, you can play all over the sandbox if you like. Until then, try, try to specialize.
At the behest of my agent, I dropped science fiction. The market was just too tight for us to get in there. I had that same problem when I was repped by Richard Curtis 18 years ago. So I slid, rather comfortably, into paranormal thriller/romance and urban fantasy. The result has been that I'm getting exactly 45% more full reads in that category. My agent was right. I upped the odds. The transition wasn't hard--I love words anyway, and it's still spec fiction. I'm strong there--I feel it and I know it.
Don't irritate your agent with trivial or non-business type communications. Unless you're really best buds. Successful agents are REALY BUSY PEOPLE. I check in about every three weeks--sometimes a month. That's unless you're doing rewrites or hammering out a contract. New repped writers have a tendency to suffocate their agents with needless questions and pestering. Give 'em some breathing room, and everything should fall into place. Contrary to popular thought, they haven't forgotton about you. Their really dying to send you that all important editor's contract. You know the one you've been waiting for all your life?
Tri