I know several tech authors who use an agent to sell their books license their copyright to publishers. Tech authors don’t need agents. You can sell to a tech publisher yourself, and hire a lawyer to evaluate any contract offered. I’ve never used an agent for my nonfiction.
Pimping yourself is work, yes. And it takes time, and you must educate yourself. But it’s not hard, or authors couldn’t do it. Before you decide to hire an agent to place your work, I suggest you read this. Some agents are transforming into publishers.
Are there good tech author agents? Certainly. Don’t ask me, I’ve never had an agent. I can’t even say what percentage of agents are good… could be 99%, could be 1%. But no profession is all good, or all bad. (Except, perhaps, clowns. But that’s a separate issue.)
If you really want an agent, if you’re convinced that you need an agent, you should know the signs of a bad agent. An agent must be your advocate. Not the publisher’s. And certainly not the agent’s. The agent’s job is to maximize your income so as to maximize his percentage, not to pay you a percent of his take from your work.
In publishing, money always flows towards the author. Any deviation from this is a danger sign.