First of all, search Wikipedia directly and via google to make sure that the article you want to write doesn't already exist. Duplicated content is likely to be merged or deleted.
The subject of a biography or a book must be notable and you need to provide proof of this. That means that they must have received significant coverage in reliable, independent, third party sources. A book may be notable and yet the author may not, and vice versa, so you may have to just add a minor "About the author" section to an article about a notable book.
Any significant facts in the Wikipedia article, such as those that may be disputed or critical acclaim, must be verifiable by other editors and readers. That is, the source of the facts must be cited in in-line footnotes or global references; and again those sources must be reliable.
Typically, a mainstream newspaper, magazine, journal or web site article or review, or another author's notable book is a reliable source. A post in someone's personal blog is probably not a reliable source. An Amazon product description or customer review are not. The author's own publisher's web pages or the author's own web site are not reliable sources. Other Wikipedia pages or web sites that mirror Wikipedia's content, are not. A trivial mention may be sufficient to establish a simple, verifiable fact; however, it will add little or nothing to establishing notability.
Any article must be written from a neutral point of view. It should not read like an advert; be used as a soap box to express personal opinion, nor contain original research.
If you're writing a biography, especially if the subject is still alive, then you need to pay careful attention to Wikipedia's policy on biographies. The requirements for biographies, especially those of living persons, are more rigorous than for other types of article.
It's possible to write an article about yourself or your own book, but generally speaking this is not advisable. You're likely to encounter problems with conflicts of interest.
You don't have to produce a complete article straight away. What you can do is create a stub article, containing just a few, basic, well referenced facts and you or other editors can gradually improve and expand on the article as time goes by.
One way to get started is to search Wikipedia to find a simple article about an author or a book and use that as the basis of your own article. You can create a personal sandbox at Wikipedia, in which to safely experiment and develop your article.
Before you create a real, non-sandboxed article, make sure you have two or three references already in place, or in place as soon as possible, to deter over-zealous new page patrollers from pouncing on the article and flagging it for deletion for lack of notability; etc.
One thing to watch out for in Wikipedia articles is "link rot". Over time, web sites and web pages get moved or disappear and the result is a dead link (though the internet archive's way back machine can help retrieve archived web pages). For this reason, and to make future verification easier, it is better not to use bare web addresses (URLs, such as "http://www.etc.etc.htm") as references. You really need to use the appropriate web, news, journal or book citation template and fill in as much detail about the source as possible, such as the title of the article, author, name of the publication, publication date, page range, URL; etc. This will save a lot of grief in the future.
Anyhow, the best of luck to you! Don't worry about making mistakes: there are other editors who will gladly help out. Enjoy the process.
• By Etienne de L'Amour ~ Google+
