Some publishers ask for sole consideration of your proposal. In my process, I have mostly given sole consideration to the publishers to whom I have been proposing. This has been largely a product of my approach: as discussed in previous posts, I feel that it’s best to write a distinct proposal for each publisher, to better match their list. Because that’s a pretty big effort, I don’t send out a lot of proposals at once. In August, I sent out one proposal that never earned any response, so I suppose that I wasn’t quite offering sole consideration on the two proposals I sent after that. Because it takes time to move from one proposal version to the next, and because the responses I did receive were generally quick (on 3 out of 5, I received a response within a day or two), I was basically offering sole consideration: as soon as I got a positive response, I focused my energies on responding to that one publisher, and not one making a proposal for another.
But I do feel like giving sole consideration puts me in a much weaker position with respect to any future negotiations. This spring, an author I’ve worked with was negotiating his book with his publisher, and he had proposed to several publishers, and had offers from (at least) two. Thus, when his chosen publisher tried to get him to change his title and other aspects of the book, he had some firm ground from which to push back. There were plenty of changes that he was obliged to make that he didn’t love making (and that, in my opinion, did not improve the book—but, of course, having worked on it, I am biased), but he had some position of strength with respect to negotiations. If I were to be offered a contract, I would have little strength from which to negotiate: basically, the publisher could tell me to take it or leave it, and my options would be to either do what they wanted or to go back to the proposal stage. Now, I could go back to the proposal stage, and getting offered a contract would certainly strengthen my confidence that my book is good enough to get a contract, but that would certainly add even more delay to this process that has already gone on for a long time.
Abstractly, I would recommend proposing to several publishers at once. But I’m not sure that I would follow that recommendation myself, just based on my own personal energy available to manage the anxiety of the proposal process. Your mileage will vary, of course: If you find self-promotion easy, then multiple submissions is definitely the way to go because you can have the added benefit of better leverage in negotiations.
Pragmatically, however, when I next need to propose a book, I think I will return to the tactic of sending brief query letters, as I did with some success—in this process, both of my query letters received a rapid response, while only one of three full proposals received a response. The query letter skirts the issue of sole consideration by being less than a full proposal—if a full proposal is requested, then I can address the issue of sole consideration. Such a letter might put you in the position of needing to write a full proposal quickly to keep up the interest of the editor who sent you a response, but if you’re under pressure because someone showed interest in your query, that’s a pretty good problem to have.