Yup. Sketchbook. That's what it says.

This where I stick all the sketches, studies, and designs I've made in the course of planning Templar. The nice ones, anyway.

You don't need to go through this section to understand the comic, but there might be some crap here that hasn't shown up in the story yet, and could, in theory, ruin the surprise later. So if you care about that sort of thing, consider yourself warned.