“Something of real quality will be recognized somewhere, sooner or later” — Mimi Jones, editor.
“Publishers are always on the lookout for a good book. This is something to keep in mind no matter how discouraging the prospect of finding a publisher is, no matter how many rejection slips you get, and no matter how overwhelming the odds seem.” — Richard Balkin, A writer’s guide to book publishing.
“Until you have canvassed at least 25-30 publishers you haven’t given your book the chance it has to get published.”– Richard Balkin, agent.
“When novice writers ask my advice about getting published, one point i can’t emphasize too strongly is the importance of being absolutely relentless about submissions. Once you’ve gotten a story to the point you think it’s worth submitting, you must submit it and submit it and submit it until someone somewhere breaks down and buys it.” — Lawrence Block, telling lies for fun and profit.
“After twenty commercial publishing houses have turned down a book, i try a small press. When you love a book, you can’t give it up.” Ellen Levine, agent.
“In the end it’s up to you. I’d say that in today’s tough publishing climate, you have to want to be a writer very, very much in order to succeed.” — Attributed to Joseph Hansen, mystery writer.
Inside My Worlds 
Perhaps the point is not so much on submitting, but knowing your book editors’ interests and building your network. I try to do this with academic journals, but I am ot sure if it is applicable to books. Best, Rafael
The most important part of submitting is submitting to the right publisher, a company that takes the type of book or a magazine that takes the type of story you have written. And building a network is important too, i agree. I selected those quotes because i wanted to make the point of not giving up, that sometimes it takes more than a couple of tries. 🙂