Your writing samples should demonstrate your abilities as a writer in general, and, as much as possible, your ability to write on the topic you’re proposing. It’s not necessary to send a copy of a complete manuscript to a publisher (they generally prefer that you don’t, as a matter of fact). As with a job interview, give them a chapter or relevant section and offer to send them the entire manuscript if they want to take a more detailed look.
One of the most obvious writing samples to include may be a sample chapter from the book you want to publish. Although it’s frequently a bad idea to write the entire book before you have a contract (particularly in the computer book field), a sample chapter or two gives the publisher an exact gauge of your ability to write well on your chosen topic. Include sample screen shots/figures/photos, rough drafts of conceptual artwork, and appropriate references to other chapters in the book. (Be sure to put your copyright notice clearly on the bottom of each page. Accidents do happen occasionally.)
Another way to demonstrate your skills is to write articles. You don’t have to do this for magazines, though; there’s a wonderful site, Instructables.com, that’s perfect for this. It’s a DIY site where you can enter instructions for anything, with photos. Take a look at it and see what’s out there. You can write stepwise DIY instructions for anything. In fact, because people can make comments about your instructions, you can also demonstrate your ability to communicate. Who knows, you might even build a following right there! (Want an example of how this works? Here’s a recipe I wrote up for Instructables.)