To open the book we need 4 keys. Each key needs 4 fragments. Each set of 4 fragments, so far, has a theme. Once the fragment is found, to prove we understand them and their greater connection to the magical world, there is an assessment, or test, before the key can be unearthed.
The first assessment: Flora
The first fragments were the names of plants, and when we found them we got a piece of a page from a journal detailing a spell.
The assessment are still doable if you wish to try them. You'll have to dig through the forum threads for the proper info you need to solve them. It's against the Book's basic rules for anyone to give you the answers, but general discussion is fine. The Book of Briars had threatened us with being turned into a toad if we shared this link, but...well it has bigger problems right now than that. So here was the first assessment.
Once enough people completed the first assessment, the first key was discovered, and the first of four locks opened,
The second key/assessment: Fauna
The second group of fragments were the names of animals. Finding each animal also revealed a piece of a larger puzzle. A tetrahedron that we had to assemble and then answer questions about. It was...involved... let's leave it at that.
The second assessment
The third assessement: Cosmos
Without a Book of Briars to tell us there is an assessment underway. Is there? Will there be? We don't know. But we've been collecting these constellations, and I'll be darned if we don't find a use for them somewhere.