Personally, the only thing I dislike more than critical feedback is unhelpful critical feedback.
-Lyra Selene
No one writes a book on their own. I think where a lot of fledgling writers fail is they try to write a book without any outside help or support. Yes, the actual writing down of words is all on you, the writer, but getting to that stage, what happens after the first draft is done, how to get from rough draft to finished draft, shouldn’t be all on your own.
New writers are often terrified to let other people read their work. Whether it be insecurity about their talent or fear that someone might steal their work (electronic copyrights are an amazing thing in this day and age so if you’re part of the latter, let me put your mind at ease). And many new writers don’t realize, or maybe don’t want to admit, that even if you’re born with the gift of writing, no one writes well without at lease some practice if not some actual education in the craft. I, myself, have a BA in Creative Writing. And let me tell you: as a freshman in college I was CONVINCED I was an amazing writer and would coast through said major.

Yeah. No.
I needed that education. I needed those professors telling me that, while good, they could tell I was turning in first drafts of essays and stories. What’s the big deal about that? Well, good is not great and if they could tell something was a first draft, then that meant they saw room for improvement. Your first draft may be good, but it’s probably not yet great and it probably needs more than just your eyes to see where it can be improved.
I try to make sure I have at least two beta readers for a book, but if I can get three or four, that’s amazing. And, I think, it’s good to have readers at different life-stages and backgrounds so I can find out what resonated with who and what falls flat and if I get the same/similar notes from multiple readers, I know it’s something to pay attention to for good or bad.
Now, maybe you want to be a beta reader for someone, or you want to develop a critique partner relationship with another writer–if you can, do, it will make you a better writer–and you want to know how to be a good reader. Critical, helpful feedback.

Obviously all writers live for the good feedback. I look for the lols, the yeessssss!, the swoons, the love this!, the good image notes throughout a manuscript when I get it back from my readers. I love those comments. When I read the critique letters I first revel in the parts where they sing my praises and tell me what they loved about the book. We are needy things, we writers, and our sunshine are those compliments and reassurances. But we want the book to be great, not just good, so we need the meat, the real feedback.
If you’re going to help a writer with feedback you need to tell them what didn’t work for you while also explaining why. Did you find your attention waning during a particularly long chapter? Did you find yourself hoping an annoying character died even though they are supposed to be a hero? Did the dialogue fall flat for you because people don’t actually talk like that, or maybe because it was a little too realistic? Were there dropped plot threads because you picked up on something that seemed important in the beginning but then it never came back around? Did you get lost in the magic system because there are no rules?
These are all things that can help a writer who’s been staring at these pages and those tens of thousands of words for the past year and can’t see these issues. Want to know why you can’t see the issues in your book? Because you know the answers to these questions/holes/problems so your mind fills in those gaps when you reread. Just like you can’t see the typos or homonyms but they’re glaring to new eyes.
Now if you’re the one with the book getting the feedback, you need to be open to said feedback. Of course the notes are just the opinion of one person and you’re welcome to take or leave every note, but you cannot, under any circumstances be offended by the constructive criticism they offer. Hopefully they’re actually helpful and constructive and unless they just say your book sucked, you need to remember that, no matter how harsh, they’re trying to help you and they took time out just to read your work and give you feedback.
I offer professional manuscript critique services for people who don’t have a writer group and there are a lot of people who think they’re ready for the critical feedback only to realize they weren’t and they crumble a little bit when the critique letter isn’t just compliments and praise. Remember, the book is personal to you and only you at this stage.

So if you’re looking to write a book, be ready to start forming your own little writing village to help get it from opening sentence, to first draft, to final draft, to publication. Writing is a solitary craft but you don’t have to do it all alone.