If you've already written a full draft, revise it so it's clear enough to be useful to a motivated reader, and should jump straight to Beta Reading.
But if you haven't written anything yet, you don't have to spend six months writing 60,000 words before asking for feedback. Nor should you. Instead, follow this process:
- Brainstorm a list of the sharp questions your book will answer for readers
- Pick one that's concrete, important to readers, and straightforward to write about
- Write just enough to answer that question
- Find someone to share it with