I want to write on a computer topic which isn't covered by many existing books out there. What is the process of getting your book published?
The answer: yes, there is such a guide. There are many such guides, at the publishers' websites.
Now on to the dessert:
If the topic is on computers, no doubt you'd like a publisher like O'Reilly.
Here they have a step-by-step guide to getting published by them. I'm pretty sure several other computer science/technology related publishers will have guides on their sites, too.
So, being the publisher the author, I think we can count it as legitimate. And it is pretty straightforward, too.
As I said before, if you're writing something that hasn't been covered, no doubt you'll want O'Reilly. But they'll ask you: why are you uniquely qualified for writing that book?
It was asked in a comment: what about fantasy or sci-fi? Answer: Tor dot com might be what you're looking for. I linked directly to their submissions guidelines' page. They don't want novels, just short fiction. But they link to Tor Books' submission guidelines. in case you have a longer work. Good luck and have fun!
I disagree with @iajrz. For non-fiction, you should write the proposal first. Publishers and agents do not want to deal with a fait accompli. Check out Larsen's book. It's a great walkthrough of the entire process.
I published a computer book in 2001, and sold 10,000 copies, so I know a little bit about this subject.