Steve Krug tells it like it is. 'People don't read nearly as much of [your book] as you think.'
Painful though it is, much of writing is actually editing: reworking sentences, cutting out fluff, converting long paragraphs to bullet-points, so that you get your point across.
Steve used all these tricks and more when the wrote the bible of usability experts - Don't Make Me Think. He wanted it to be readable in a two-hour plane journey, because that's about how long his target reader would be able to give it. And to achieve that he did a lot of 'throwing stuff overboard'.
Writing, says Steve is like usability: 'it's all about 'keeping the user in mind and trying to be as kind to them as possible and trying to make it as rewarding an experience for them as you can.'
Invaluable, practical and refreshingly sane advice whether you're writing a book or a page of website copy.