1. Publishers know best
Publishers may have better access to information about trends, but may not necessarily know more about your book and market than you do. They do not normally consult authors on marketing issues such as cover design, but that doesn't mean authors should refrain from speaking up and asserting themselves when they feel strongly about editorial and marketing decisions. Remember, you are the expert on your book, the person who's most passionately committed to it.
My experience: The editor and I discussed the changes she requested. Where I put forth a convincing case for leaving something as it was, she went along, and in other places she convinced me that the changes would make the book better. In some instances we arrived at compromises which satisfied us both.
2. I don’t need a literary agent
No matter how good a marketer you are, you need a literary agent to protect your interests when entering into a contract with a publisher. Many authors are so happy to sign a book contract that they don't realize until it's too late that, for example, they've signed away rights they should or could have kept. Agents know which clauses to look for in order to avoid pitfalls like having to submit future book proposals to the publisher of their original book without a specified time frame for acceptance or refusal, among other things.
My experience: Authors acting on their own tend to accept the publisher's first offer, whereas an agent can almost invariably negotiate a better deal for you. Mine did. I was also able to retain rights the publisher initially wanted included in the sale.
3. My book will sell itself
The chances of your book being 'discovered' in the bookstore and becoming a resounding success are slim to none. It's not the publisher's job to promote your book; it's yours. Research all the methods of marketing your book and work at it. Luckily, in these Internet times you can promote your book on a limited or nonexistent budget. Embrace the search engine: more and more readers turn to Google before going anywhere near their favourite bookstores. Blogs, websites, review exchanges, contests, interviews, social media tools such as Facebook, Twitter and MySpace, newsletters and cybertours are your friends.
My experience: The editor requested my marketing plan long before the book's publication date. I had already done the research and so I was ready. I'd started a writing blog one year ahead of the publication date in order to establish an Internet presence and to network with other writers, and had entered info about myself and the book in as many Internet directories as I could, among other things.
4. All my friends, family and acquaintances will buy the book.
Prepare to be shocked: total strangers are far more likely to buy your book than friends and family (who generally think they're entitled to free copies). And on the topic of free copies, yours should be given away freely for promotional purposes only. Friends and family (with the possible exception of parents) should buy theirs.
My experience: I gave away most of the author copies of my first book to family and friends then had to buy promo copies out of pocket. Now I know better.
I've been researching this industry for years and I continue to learn all the time. Knowledge is power in every sphere, and especially so in the world of publishing where so many potentially damaging assumptions abound.