So when it comes to how you know if your book is any good, I'd say that if you can hire an experienced pro to tell you, you'll *actually* know. If you're going it alone, querying will tell you. I didn't know if I was technically ready to query, but I was doing it anyway.
I suggest sending out 5 queries to a variety of agents on your list. If you get nothing but form rejections, read more QueryShark and rewrite your query. If you're getting requests, keep querying and wait for responses. But what do those responses mean?
Decoding query responses:
1. Not a good fit= generally a form rejection
2. Agent offers compliment/any personal advice = getting close!
3. Agent suggests specific changes and says they'd look at it again = so close!
4. Agent wants to have a call to talk about it = SCORE
Still not sure if your book is good/if your edits are helping? Ask yourself:
* What does the protagonist want?
* What is the main conflict stopping them?
* What subplots complicate it?
* What is the character arc?
* Does the end satisfy the beginning?
* Can you write a hook or blurb for your book?
* Are there any parts you consider boring?
* Would you tell someone it gets better in chapter 3/whatever?
* Are there weak spots you hope an agent won't notice?
* Are you proud of it?
Still not sure if your book is any good? Go ahead and write a 1-page synopsis, which many agents want as part of your query. Is there a definite plot-- this happens, which causes this, and then this? Or are most of the verbs passive-- she learned, he discovered? Plot= vital.
Check your first chapter. Is it a character experiencing a normal day in which nothing happens, walking around a room thinking about something, or describing themselves in a mirror? Those are all red flags. Consider the instigating factor and rewind one scene. Start there.
IME as a writer and teacher, many 1st chapters are the writer thinking out loud and waiting for something to happen. If that's in your first draft, cut it and decide where the book really starts: in a moment that shows character and worldbuilding while kicking off the plot.
One issue I see in books at the querying stage = the writer is ramrodding the plot when the main character should be driving it. Your protagonist should be motivated to do something, and their agency is what makes things happen. The world and antagonist should impede them.