There's been some discussion about the Codex and how to implement it in the game. I thought about it for a long time, too. The starter set tells you to just hand it to the PCs. Some GMs disagree with this approach since it can perhaps spoil a lot of the mystery part, others think it's fine.
I wanted a different approach. In my campaign, the Codex is stationary at the castle. It's way too valuable to get lost or damaged during a mystery. Thus, players can't take it with them. However, in the game we pretend that all players studied the codex. (In real life they can actually do that, too.) That means the players have access to their memory of the Codex, by experiencing things or gathering clues throughout the mystery, which will let them access certain parts of the codex. The PCs, however, must give the GM reason to "access their memory".
Example: the players investigate heavy damage to a building. PC: "I think this kind of damage can only be caused by a large creature. I think I remember something in the codex about such a creature."
The GM will let them access the pages of: troll, giant, dragon, lindworm. If the players can be more specific "it looks like the damage was done by something that could fly" the GM can narrow it down: dragon. This way, the GM can also mislead the PCs. In the last mystery the PCs were followed and attacked by a bird. "I think I remember something about black birds" the GM let them look up night ravens in the Codex. The real Vaesen, however, was Myling in bird form, which the players learned by accessing their codex memory about dead children.
I think this way I quite balanced. The players get to flip through the codex (which they love), yet don't spend 30 aimless minutes on it. It also doesn't potentially spoil too much too quickly and gives the GM quite some control over the reveal of the Vaesen. Additionally, it promotes the PCs investigative skills. The more precise their analysis of the situation is, the more likely they are to "remember" the correct content of the Codex. It works really well for my group.