I have been blessed to be a published author both by the traditional process of an agent and a publishing company who managed the editing, marketing and every other aspect of the book process; and I have self-published where I managed every aspect of the process. From both processes, here is what I have learned:
1. Book publishing is about sales...not your great writing or your fabulous story or your credentials. Having a target audience that will buy your book and a great marketing strategy is the most important aspect of book publishing. There are many, many wonderfully written books with the most creative plots, inspiring stories, or great advice that do not sell. Publishers want to sell books and even self-published and indie authors are all about sales as well.
2. Professional editing is worth every dime...even if you aced your grammar course and love diagramming sentences, you still need an editor. I find it extremely difficult to edit my own work, despite the fact that I once taught grammar and creative writing classes.
3. The more eyes or your work the better...don't hold on to your writing as if it were high-level, classified information. Sure, it is your intellectual property, but every writer, no matter how great, benefits from feedback, the more the better.
4. When writing fiction, show don't tell...it is always better to describe a scene or create dialogue rather than straight narrative.
5. Read books on writing...they help. My favorite is Bird by Bird by Anne Lamott.
6. Join professional writing associations...even if you only attend one conference a year or page through one journal, you get to know the industry and feel a sense of belonging to a noble profession.
7. If you have self-published, don't spend money on PR/marketing firms...unless you have a huge advertising budget and money to burn. Yes, PR/marketing firms can get you exposure, but they cannot sell books. Only you can do that. The ROI (return on investment), especially for new authors without a convenient audience, is very low. Once you become a best selling author, you may need a firm to take you to the next level. However, if you have self-published, starting out it is best not to spend your money with marketing agencies. If your work has been traditionally published, you should still expect to do a great deal of work promoting your book.
8. If you feel you must invest in some kind of marketing, spend money on advertising with book bloggers...they are all about books and folks who visit their sites love to read and are seeking the next best read.
9. Reviews sells books...positive reviews are great, but even negative ones help your writing and sell books. The best gift you can give an author is to write a brief review (you don't have to be a great writer--a thumbs up or thumbs down suffices) and rate the book on sites like Amazon and Goodreads.
10. Be realistic about book sales...even though publishing is all about sales, writing is about communication. Even if you have twenty good fans, that means there are twenty people you have communicated with and twenty ambassadors for your work. Remember the majority of self-published authors sell less than 100 copies. New authors with traditional publishers sell about 2,000-5,000 over the lifetime of their book. Best sellers are in the range of 30,000 or more. If you are a published author, whether you've sold 20 or 20,000 copies, the joy of writing always remains.That is the lesson I have ultimately learned.