I stumbled across this blog post by Eddy, who shows off some photos of his journals, and talks about how he uses them:

Since I started using notebooks, I’ve kept all of my projects in one notebook, instead of using multiple notebooks for multiple projects. Usually I have had only a couple of things going on at any one time, so it’s easy to keep track. However, as I started taking notes for the Sherlock Holmes essays I’m working on in my spare time, I found it irritating to try and find my notes when I was writing each essay, or when I needed to compare thoughts between a previous story and the one I was working on, because it might be days or weeks between me reading a story and taking notes and the time I actually get around to writing the essay. So I bit the bullet and decided to use a project-specific notebook for the first time. I found an empty ruled notebook upstairs in one of our filing cabinets. It’s ruled on one side of the page and blank on the opposite side – a weird combination (and probably why it was never used for anything), but good for me to help easily separate my various topics.

See more at eddyfate: My Three Journals.