Before you finalize your query letter for any work of non-fiction, you should google your subject's name and see what comes up. Generally it's the Wikipedia page. Read it.
If you're querying me about a biography, that's exactly what I'm going to do after I read your query. The reason I do that is because one of the things I look for in a proposed project is whether there is something fresh and new to say about a subject: new information; new analysis, new sources.
If you've queried me for a project and you fail to mention some of the basic things I can find in a quick google search, particularly the negative ones, it does not bode well for the quality of your research.
And if Wikipedia and the generally available sources are wrong? Hurray! This is exactly the kind of information you WANT to include: "everything we think we know about Subject X is wrong, and my book will fix that."