My mom says that my books are really good and that people will enjoy them. I know parents say things to their kids to keep their hopes and dreams alive, but I want to know: How do you know if your book will really be enjoyable for everyone? How do you tell if your book is ready for the world?
You need to run your book past readers who are capable of assessing it objectively. That may or may not include your mother, but it should certainly include more people than just your mother. Look among your friends and friends-of-friends for people who read a lot of books in the same genre/style as yours. Ask them to give a critique. You are not obliged to follow what they say, but be open minded.
Do not let this process delay too long the real test. Submit it to a publisher. When it is rejected, as most books by unknown authors are, submit it to another publisher. And again, and again. Or, if you can afford to, self-publish. It is much easier and cheaper to do this using electronic publishing than it used to be. To self publish is to submit your work to the ultimate test: the reading public. I am afraid that there is no real way of being sure that your book is ready for the world except actually showing it to the world.
Criticism is inevitable. As The Thom says above, you can be certain that your book won't be enjoyable for everyone. To be honest most writers struggle to find much of an audience and would even welcome a bad review on the grounds that at least someone is paying attention.
This advice isn't intended to crush your dreams but to give you a better chance of making them reality. Rejection always hurts. But if you really want to be a writer you will take the risk. I've read that hardly anyone who has written a book regrets doing so.
Speaking personally, despite having a publications list as long as your arm in a non-fiction field, I really struggle with a tendency to delay and delay until I am sure my fiction is "ready" by which I mean "perfect", a thing that will never happen. So I'm talking to me as well as to you!
In general, I think @lostinfrance's answer is excellent, this is just a supplement to that:
You can't necessarily ever know if your book is ready or not in terms of content, but you do need to make sure it is ready from a technical point of view. If it has any spelling, grammar or formatting issues, it will never be given a fair read by most people (particularly professionals in the industry).
There are a number of good books out there (you should be able to find some at your local library) that cover all the basics of formatting a manuscript correctly for submission.
As far as content, it's good to get an outside opinion (not a family member or best friend) but ultimately you need to make the determination for yourself --any given person is just giving you his or her own reaction, another reader might respond differently.
As others have noted, at absolute minimum you must have someone look over your work for technical mistakes (spelling, grammar, sentence structure, vocabulary). You must do this even if you self-publish. No one is going to read a book so badly structured that it's incomprehensible.
I think it is also imperative to have an editor, someone who is at least moderately objective and who will be honest with you about plot and character. It helps to have someone familiar with your book's genre, but for me it's not a requirement, because you also want readers who are outside your niche to find your story appealing and understandable.
Also, remember that even the most loved books have detractors (and conversely, the most hated books have fans). Your goal should not be to please all of the people all of the time, because that will never happen, but to please some people enough that they recommend your book to others because they think it's worth sharing.
I do agree with some of the previous highlights but my opinion is different, similar, but different.
Write your book and your story for you. Yes, ensure that it is technically ready (Lauren Ipsum expressed this subject perfectly). But do not change your story for anyone just because it will fit the masses. That's the first step to hating your work, because you're no longer writing it for you and because you have a great story brewing in your mind.
- Write your story for you and for you alone.
- Don't listen to the sad people that says your story needs to apply to the masses. While this might seem reasonable because you want it to sell, money will not make you happy. (Writing will because you, like the rest of us, love it. Can I get an Amen?)
- Do your writing for the pleasure you get from writing. To feed the creative side of you and all it's passion. Put YOUR heart, feelings, creativeness into it. (Like you already have, I'm sure.)
- Publish the book if you want and consider only one persons opinion if it's ready or not. You. Do you feel that it's ready?
Most importantly! (This concerns point 4.) Don't let a fear of not being accepted stand in the way of you sharing your story with the world. And do not overly obsess about how your sentences are structured and what would work better than what you had previously. This can be done for years and years and you'll never get anywhere.
With this said, Proof-readers are a good thing. Let people you trust read your story and only rarely let them affect the outcome of your characters and the plot if something really doesn't make sense with YOUR vision of YOUR story. Maybe someone goes out of character and does something the reader will feel that character would never do? Then you need to consider if that has to be changed or not. Maybe the character is just going down a bad path? You catch my drift. In essence it's up to you as the writer and creator.
As my rant is coming to an end I will finish off with a summary: Write your story and share it with the world if you want. You can never please everyone and don't try, because if you do it's no longer your story. You will rarely be happy with the outcome if you allow other people to define your story with their views and assumptions about what your story should be. This is as true in writing as it is in life.
Write your own story.
A book is ready for the world when the composition is complete and correct (grammar, flow, the story finishes, etc.) and when you are satisfied with it. Putting it out to other readers as a pilot is part of putting it out to the world in my opinion. When your book is out in the world, you get feedback on it which both defines and determines if your book is ready. It's an iterative cycle. If the book isn't ready and can still be edited, then make the appropriate changes. If the book can no longer be edited, it's a bummer but not a loss. You can take what you learned and apply it to the next project.
The feedback from putting your work out there will range wildly in both usefulness and hurtfulness. Take the positive stuff, whether it is useful or not, and hold tight to it. Let it lift you and encourage you. Let it push you upward and outward. It is one of the sweetest things in life.
For the negative stuff, if it isn't constructive or useful (e.g. "this sucks!" or "why would anyone read this?") discard it. Forget it. Erase it from your mind. If the criticism is malicious or too generic there is literally nothing you can do to fix it. All it will do is weight you down. Don't allow that to happen. Ever.
And lastly, we have the critical but useful stuff. For better or worse, this is the biggest area for growth and is a valuable indicator of readiness. This is when people say stuff like "this character behaves weird to me" or "there was way too much description on that doorknob" or "I have no idea what's going on right now". It may be hard to not take personally, but this is really the good stuff. It's what people honestly think and feel about what you've written and is great way to become aware of problems you didn't know were there and to get a feel for how your book is being received. Through this mirror you can fix what needs fixing (and not all of it will) and put it back out there for another round.
The cycle continues until either the world wears you down or you convince the world your work is good.