From notes into acutal historical nonfic...

Fiction or nonfiction? I imagine those are different animals.

Assuming from the post it's a non-fiction project:

First off, make sure you have a sense of the overall thesis or through-line of your book. Organization is the key to turning a mess of facts into something meaningful, as you know from your experience writing articles -- a book-length project adds complexity and scale, but the fundamental process is very similar.

A jumble of bullet points sounds like you have a lot of facts and ideas, but not a clear structure or argument to hang them from. You may want to play around with brainstorming exercises (Google can easily find you a lot to try) that can help you group the facts-and-ideas into clusters that make sense to you, and then to identify the higher-level points that the clusters link to. A fish-bone diagram may be useful!

Since you're comfortable writing articles, you might consider using these idea-clusters to chunk your book into article-sized segments and tackling them individually. If some seem easier or more attractive to write, go ahead and write the easy chapters first! Don't stress about the high-level argument or focus -- you can always work on that in the revision stage, and it's entirely possible that the detail work in the chapters may cause you to rethink your original point so you'd have to redo it anyway afterwards.

Is this because a book is big, long scary thing and the idea of working on it puts you off?

(If that is the case, it might help to break it into smaller manageable tasks. Think of it in terms of chapters, or just focus on what you are going to do today.)