Re: Birol's experience. I had almost the exact same experience and the editor read my shorter than the guidelines suggested work. She said thanks, but no thanks, but please submit if other work of yours meets our criteria. I find those responses to be above the call of duty and appreciate them very much.
Re: query letters about nonfiction books. I shopped a query around to one agent and one editor a couple of years ago and each one responded personally. The agent had just gone out on her own and wrote glowing comments about my query but felt that shopping the book wouldn't be worth it for her. She told me that because of the idea (advice on how to give gifts to kids) and my development of the idea, she thought I should go directly to some small to mid-size publishers (Nomad being one of them I must confess). We exchanged a couple of more emails and I thanked her profusely (not obnoxiously though, I think) for her input. How kind of her to give me that time and feedback.
The editor was less enthusiastic. I followed up her response because I thought that while she raised a good point (she thought everything I wanted to write about could be found on the web - and I thought, what can't be found on the web?), I thought maybe the problem was that I had not delineated or distinguished my idea well enough. So I emailed her back and asked her if she wouldn't mind being more specific re: what sites she finds to be as complete as what I was proposing. She bristled a bit, said that there wasn't anything really, but that when she needs advice about buying for kids, she goes to Amazon, puts in some ideas and that generates enough gift ideas for her to choose from.
Well, after that, I realized I would be seen by her as a pest, I was very lucky that she stuck with me for two or three rounds of email (I had a personal reference to her) and I thanked her profusely for her time. If I had to do it again, I don't know if I would have followed up, but again, because of the personal connection, I kind of thought at least one followup question was ok.
Due to other events in my personal and professional life, that nonfiction book's proposal is behind me on some shelves, about 80% ready to go to an agent in NYC who will read it whenever I'm ready (we've been communicated for over two years and she's been a big cheerleader of my work).
The lesson to me is that when an agent or publisher indicates that they take email queries for nonfiction book proposals, send the best query you can produce. If they take snailmail queries, do the same. But never lose sight of how big the field is and how busy they are. Be polite, be specific, be professional. And pray.
Hope this helps. Thanks for letting me share.