Format Magazine is out with a great page on pitching editors:
https://www.format.com/magazine/resources/art/how-to-get-published-advice
(external link)
They advise most of what I’ve recommended over the years but I’d add a few things.
Consider making two or three pitches in an e-mail. An editor knows in a sentence or two if your project is intriguing, why spend four or five paragraphs developing your thoughts when one or two will do? Since you have the editor reading, pitch another topic.
Fewer than one in ten cover letters leads to acceptance, get efficient at writing them and realize they are as hard to craft as the article you are proposing. Learn to accept rejection, and what is even more frustrating, get used to absolutely no acknowledgment at all.
Consider carefully what you might get paid. If the magazine pays only $100 or so, is it worth your time? If the article will require travel, your own photographs, and 2,500 words, it may make more sense to pitch another publication that will let you at least break even.
The time you spend writing for very little could be better spent making pitches to a magazine that will reward your effort. Unless you are resume building or just enjoy writing on a particular subject, always angle for better paying work.
Regarding book proposals, I’d say to query with a one page letter before writing a full proposal. A complete proposal will take you at least a month to write. See that a publisher is interested first rather than commit to what might be a doomed project.
