I certainly haven't written any books, but I've written a lot hardware reviews, some software/programming articles, technical documentation, and I'm doing my second public PHP presentation this month.
When I'm doing reviews, I always start with an overview of what I'll be writing about: I cover the scope and purpose of the document, and what I hope to accomplish before it's done. Then the reader can bug out if it isn't in their interest, instead of realizing four pages deep that they don't care what I'm talking about.
Planning is key, always create a rough outline of what you are covering before the document gets wildly out of scope.
Always back up an argument with sound factual data and sources. An opinion isn't much good if there is no reasoning behind it.
If you're listing code, make it clear what versions and platform you are using.
Don't be overly talkative, but have some personality. Half the tech books I pick up end up back on the bookshelf because they either:
a) Have no real content and lots of fluffy chatter.
or
b) Have lots of content, but are so cryptic and/or boring it makes your head spin.
Obviously, use proper grammar and spelling whenever possible. Your editor will love you, and you'll spend less time dealing with revisions.
Strunk and White's The Elements of Style is a classic reference for proper usage and common mistakes in the English language.
Most of this is probably obvious to you anyway, but hopefully there is something here that can be of use to you.