Clarity in Writing is vital. If meaning is obscured, the reader will not understand, or worse still won't care and put the book down. This is teh same when writing a book, even when writing a fiction book or non-fiction book. What makes writing clear? Really it boils down to three elements.

1) Spelling and Grammar:

When writing a fiction book (I'll use this as an example) you need to understand that there are so many aspects to writing.  I think that beginning writers overlook many aspects, one of them being Spelling and Grammar.

This is a most often overlooked piece of information, if for example, the writer was to write "Do you?" it means something different from "you do?" the context might help, but even then, sometimes it is best to look at both grammar and spelling. A Good book is Elements of Style by Strunk and White. It has, according to wikipedia in circulation for nearly 100 years, (http://en.wikipedia.org/wiki/The_Elements_of_Style) and this always is a good book to have on hand. Thin and inexpensive.

You can also get editing services done on a work in progress. It's simple to do as well as all that is needed is to google. I prefer some of the more basic sites, the ones that check for spelling and grammar, and words, but that you can also add content to as well.

Spelling and Grammar are the Foundation. There's more.

2) Content:

Although many rate Content as the most important, if spelling and grammar are horrible, the writing will not be clear. If the content is off, the reader will not read it, and will feel betrayed by the writer. If the content is supposed to be a mystery, and it becomes a fantasy with a hint of mystery is isn't a mystery.

If the primary character within the span of two pages changes so drastically, and thereader didn't have an idea this might happen, or worst still expected something else, the writer is lost. Writing in whatever medium such as blogs or novels, or websites require good content.

Go to any blog, (such as www.blogger.com or www.wordpress.com) and anyone can see if the content doesn't seem the same as the rest of the blog, it probably isn't something that the blogger to time to think of. Writers are judged by content, and not much more.

3) Style:

Style is as important as content and spelling, as this is the the area where a writer must shine. It's not called elements of Style for nothing. Let me give some examples: Stephen King, Terry Brooks, Harry Turtledove, they are all writers, but we associate certain styles to each.

Stephen King "the Master of Horror" his style is direct and consistent. He grabs his readers with visual imagery.

Terry Brooks "Master of Fantasy" Although this one might be one people would argue, I bring it up for one reason: I read his writing. His writing is somewhat over the top, and in the 3rd person omnipotent. So rare that he changes this style, it often comes as a surprise to the reader, and hotly debated on his website because of it. (www.terrybrooks.net)

Harry Turtledove "Alternate History: Turtledove takes the what if to a new level. He almost always includes a religion or two to his writing, and a "romantic couple." Although never graphic, there isn't much a sense of mystery as the focus is often on the what if, and some minor character dealing with these changes.

Clear, Clear, Clearest:

Any writer needs to have these three thing to keep their writing at the finest level. The better the editing on the writing the better it will be. There is a trust between readers and writers one which cannot be broken.