After a book is complete, it’s always a great idea to send a copy of the manuscript to a literary agent - unless a publisher is already waiting for you to send the manuscript.
Numerous publishing houses, especially the larger houses, do not accept unsolicited manuscript submissions without agent representation. That means without an agent, DO NOT send a manuscript unless a publishing house contacts you and asks you to send a manuscript. If you send a manuscript to a publishing house that is not accepting submissions, without the publishing house requesting the manuscript, a receptionist will throw the manuscript in the trash without reviewing a single word.
A publishing house is soliciting a manuscript submission when the publishing house contacts an author and requests a copy of a manuscript. A publishing house requesting a manuscript submission typically happens when a small press, or an individual, publishes a book and the book becomes commercially valuable.
An author should always submit a book to a literary agent as soon as the book is complete. It’s best to know your options, and a literary agent typically knows the best.
While agents are reviewing your manuscript, take six months to edit your manuscript. Read the story backward, and then forward.
I’ve stated the following several times, read your story backward. Any editor that knows how to edit will admit editing a story backward is the most efficient way to edit.