Query to agents or direct to publishers?

Is it very rare for a publisher to read unsolicitied manuscripts. Your best bet is to start with querying agents. They have access to doors we can't even find. They have established repuations within the industry. They know exactly which editor would be interested in your book and how to tailor a proposal package to capture that editor's interest. In my experience, my agent has been invaluable, particularly during the contract negotiations, which I would not have understood without her experience and expertise.

Try going to agentquery.com and searching for agents who are interested in your genre. It's a great resource.

There are a few publishers who look seriously at unagented books, but their number is not great--and even at good houses that do this, such as McAdam/Cage, the slush pile is the lowest-priority.

I'd certainly look into getting an agent first, though even a good agent isn't a guarantee of anything.

May I politely point out...

that a query letter to a specific editor at a publisher is not an unsolicited manuscript. If you do sell your work and self to the editor in that query letter and they ask for a partial or the whole book, it is not unsolicited, nor will it land in the slush pile.

It's not American to do this, I know, but sometimes going direct to a publisher is the right thing to do.

It depends on many factors. Some genres, like science fiction and fantasy, have a lot more publishers willing to accept unsolicited manuscripts. Small or University presses also do. If your book is a niche market, a small press could be a good fit for you and you wouldn't need an agent (though you'd probably want to find someone knowledgeable in publishing contract law to check any deals before signing).

With a mainstream novel, you'd probably like to see the book sold and on book shelves everywhere. You're better off in the long run finding an agent. Agents do a lot more than just get your book into the hands of book publishers. They handle the deal, the legal aspects of publishing, handle royalties and taxes, and help you build you career. Many help you revise to make your book as good as it can be (my agent is a gem in this respect). They handle the business side so you can do what you love -- write.