One of my phenomenal interns emailed me this morning and asked if it was OK to read only the partial of this manuscript that she thought looked really good.
"Why don't you read the full, if it looks so good?" I asked, drooling at the prospect of something kicka$$ soon to cross my desk.
"Oh, he hasn't finished the book yet." She replied.
Y'all finish your books before getting our hopes all up and stuff! This goes for all fiction and for memoirs. Finish the book, let it sit a couple days, edit it, have beta readers read it, and take their suggestions to heart. Let it sit a couple more days, do one last pass, THEN send.
If that seems like a lot of work, consider the competition. There are authors out there that are doing that, and they get signed. And then they go out on submission faster because they've done work on the book. And then your agent feels happy because they didn't have to re-write the book for you.
I'm going to do my job: find editors that will want this book and sell the crap out of it. Hell, I like editing, so we'll do some of that too. But don't you cop out on your job. You should be of the (honest, corroborated) opinion that your book is ready to sell THE DAY you query it. Get used to editing. You'll do a lot of it.
Just a note, nonfiction can be pitched and sold on proposal and a sample chapter. So that's a different case. Nonfiction authors do their pre-query work on the platform side.