Getting a literary agent before you write the book (on the basis of a proposal and some sample material) is actually usual in nonfiction. That’s what happened with my book — I secured my agent with a one-page summary and a meeting at Soho House, having not written anything else yet! After we signed a contract and she was representing me, she worked with me to shape a very long (50+ pages) proposal for my book, which included an outline; information about related and competing titles; ‘about the author’ section; a marketing plan with lists of your contacts and an idea of your platform; and some sample material.
On the basis of that I got the book contract with a publisher (and yes, it is difficult, although not impossible, to get a book contract without an agent), and I got an advance that was divided into 1/3 on contract signing, 1/3 on submission, and 1/3 on publication (that’s pretty standard too). I give lots more details on the chronology as it unfolded in My Publication Diary, published in the Writing Cooperative.
In fact, editors who work with nonfiction don’t usually *want* a fully completed book to land on their desk, because they want to work with you to shape it. For fiction, on the other hand, you usually do have to write the whole book before securing the literary agent and then the publisher.
That said — make no mistake — putting together a strong proposal that will sell IS in and of itself hard graft, so you are 100% spot on about there being no way round it, only through it. (By the way, when Rebel Writers Club gets properly going in January 2020, I’ll be sharing my proposals and my proposal tips, and I hope other folks will do the same if they have examples.)
Thanks for writing this Habbi!