Let's first distinguish between genres. Have you written fiction or nonfiction?

Nonfiction books are easier to shop around. If you wrote a book about gardening, find yourself a list of publishers that specialize in that area. If you wrote about dieting, food, sports, hunting, computers, math, knitting, traveling, find the publishers who make their money off of those things. They're out there. Try California first. There are six times more small publishers specializing in small nonfiction niches there than anywhere else. They work mostly out of their homes, which is good news because it means that they are frugal, always a good omen in the publishing industry. The bad news is that they average no more than seven new titles per year, which diminishes your chances of being accepted.

There is one exception to the notion that nonfiction books are easier to shop around. Christian books are not easily accepted, despite the fact that there are tons of Christian publishers around. Christian publishers tend to be very selective, sometimes because they are strictly denominational, more often because the supply of new works far outnumbers their demand.

What if you have written a work of fiction? If you are a first-time author, chances are that you have done exactly that. Most first books are fiction. The main reason is that most new authors trust themselves more with being a story teller, with being an observer, than with being a participant in their story. In reality many first fiction books are at least in part autobiographical, but their authors want to play it safe and prefer to hide behind one or more characters. There is nothing wrong with that. Baring one's soul in writing is difficult enough as it is. Only younger authors are more comfortable than middle-aged authors with also revealing what they think their true self is.

Are all fiction books difficult to market? The short answer is, "yes". The nuanced answer is, "not always." Nora Roberts, Tom Clancy, and Stephen King have no problem getting their fiction published. But you are not them, not yet. A second caveat is that science-fiction and fantasy writers have it easier. It's unfair, but such is life. As a rule of thumb, the quality bar for sci-fi and fantasy is a lot lower than for all other fiction. Therefore, beware of published authors who are self-crowned writing experts. When they tell you what to do and not to do in getting your book published, always first ask them what genre they write. If it's sci-fi or fantasy, run. They have no clue about what it is to write real-life stories, and how to find them a home. Unless you are a sci-fi or fantasy author yourself.