Plus there's humour everywhere else in the game. It's taking the piss from all the overly serious dungeon romps. Hint; if 80% or more of your game is set in underground labyrinths with insane architecture that wouldn't be useful to live in, with no apparent sources of water or food, chances are your game probably shouldn't take itself too seriously. You're already killing cat girls by the dozens at that point.
So why stop at the codex? Especially given that, like as not, any such book in-game would be getting penned by some poor bloke who would doubtlessly rather be doing pretty much anything else. I mean Hell, what good is a codex going to do your typical dungeon-dwelling adventurer? What few of them can read probably wouldn't bother to do so anyhow. Heck, ever notice that Gorgons aside, there don't appear to be any female adventurers? It's 'cause all the races are sending their stupidest men into the dungeons to get killed over 30-35 gold or so. Even the success rates of veterans don't justify that kind of risk, let alone the piles of corpses novice players build up.
Don't be the "stop having fun, guys" guy. :p