When I'm not writing novels or short stories, I develop software to keep the lights on and the fridge stocked. A key stage of the software development cycle -- the steps taken to engineer a software product and release it to the world -- is "beta testing."
There comes a time when the development work is essentially finished, but the product is in a rough, largely untested form. The engineers have run it through rudimentary tests, and the quality assurance team has subjected it to some of their battery of standard tests. But, it is still imperfect. That's where beta testing comes in. You send a series of these rough versions to your best customers, who have bravely volunteered to be Guinea Pigs, with the understanding they will use it every day and ruthlessly report back on its bugs.
This concept is also valuable to writers -- especially for novels and non-fiction books. Once we have edited our first draft into a readable second draft, we are at the stage where we need honest, knowledgeable "beta readers." These are people you trust, who will dedicate themselves to read through your manuscript, cover to cover, without delay, and make detailed notes on its flaws and charms. I have been trying to find a few "right" beta readers for a long time, but as yet without success.
Family usually doesn't qualify because they will feel compelled to say nice things to you or, conversely, they still hold a grudge from when you were 11 years old and will rip your work apart just for sport. If they say, "It's great. I loved it," are they really cringing inside about how lame the story was and how it's better suited for insomniacs?
I've tried local librarians and booksellers, but they insist they and their staff are overwhelmed by required reading. Writer friends ought to make good beta readers because they have more knowledge of the mechanics of writing. But a writer whom you barely know, and who doesn't care if a friendship ever develops, may be the ideal beta reader. They have no axe to grind either way. They'll let the chips (and worn out clichés) fall where they may. And you should get the honest feedback you really need.