The book looks like an encyclopedia. There is writing inside and what look like chapters. There are charts and captioned illustrations. The drawings show plants, creatures, and landscapes. However, the "alphabet" for the writing is made up and the illustrations are all fiction. For example, one page describes creatures that look like eyes on the surface of the water.
When I flip the pages, I pretend that I'm a kid who doesn't yet know how to read. I look at the pages trying to make sense of what the pages are trying to say, even though the content is fundamentally nonsensical. I try to analyze the charts to identify any trends or patterns.
This book helps remind me of the wonder in learning and wanting to form conclusions. I like thinking about the possibilities of what the book might be trying to convey. Eventually, I have to come to terms with the fact I will not understand the text. It cannot be understood (save perhaps by dedicated cryptanalysis).
On another level, the codex helps me think about literary analysis, and what is truth. If I draw a conclusion, was that conclusion suggested by the text or something I identified myself? Is my conclusion truthful? If so, is my conclusion truthful to the text itself or would it also be true in a world described by the codex?