Hi there, I write non-fiction, history to be precise and I've honestly seen books ranging from all sorts of lengths. The book I'm reading right now is about 600 or so pages, yet I've read books that are also about 200 pages.
It depends on your focus, are you doing a broad history of the topic or are you focusing in a lot tighter on one issue, such as government structure, the role of women, the economy etc etc.
As for the credentials, I've had a lot of experience writing academically about government and history and am studying toward a Classical Studies degree. By no stretch of the imagination is my stuff perfect, but it's all about your voice as well. I've been told my work is very readable and even people who wouldn't normally read history have taken an interest in what I'm saying. It's all about how you word your argument that works sometimes.
I blog about how I write and if i'm having an 'off day' i'll rant there, if you want to check it out feel free. What sort of topic are you looking at? (I'm interested in your project!)