I generally assign a theme to a new sketchbook. Often times it's a subject matter I want to focus on, sometimes a new medium, sometimes a new methodology. I usually base this choice off of whatever I struggled with or got bored with in my previous sketchbook.
My previous sketchbook had me focusing on full-body pictures and gesture drawings, as I wanted a better understanding of where I was placing anatomy before rendering it up. Additionally, I was relying too much on my classic crutch, head studies. I got a lot of success out of this gesture-oriented sketchbook, but with one problem: practically nothing rendered into fullness that I felt like posting!
As a result, I made fully rendered, finished pieces the focus of my current sketchbook. And now I've got way too much to scan! But it's almost finished.
Here's a preview, an attempt at Bouguereau-esque imagery of genteel girls doing girly things in garden settings. What a tool that Bourgeois Bouguereau is.