As for the ability to absorb knowledge gleaned in reading or listening...given that most Knowledge-skill focused characters tend to be the high INT, wizardly or bardic types (or some clergy ) I feel rather save that they have read vast amounts of seemingly trivial, yet interesting or simply "required reading" stuff as part of their studies. This assumes, that such characters are of a certain age and maturity, of course. And, as the GM, feel free to limit advances in specific skills which weren't actually used or trained at all recently - I mean how big is the chance of the wizard/scholar/bard studying and memorizing "your-olde-book-of-monsters" while doing a non-stop raid through the halls of the giant kings ? Nil ?
Two other points might factor what maybe learned by applying knowledge (xxx) - for one, the GM decides just what snippets the character learns (which might not be the weakness he was looking for ), with obviously useful and 'well-known' information being handed out first - like trolls regenerating and their dislike for fire and acid. But does he know they can rend or if they have reach ? Or in what numbers they usually occur ? Or maybe that knowledge has already become outdated - say in case of a lost culture, who once were pacifists jungle dwellers, but have recently embraced a new faith featuring demon worship, cannibalism and extensive use of poison.
Second, figure in what the character knows - and hand it to him written on a note or take him aside, to tell him in private. Point being - that PC knows, not everyone else, he has to communicate that knowledge first (not necessarily easy on a turbulent battlefield ) - and likely spend standard or even full-round actions explaining things to his comrades, after all a single combat round is only six seconds worth of time. If he starts spreading meta-gaming knowledge out of character, smack him down for it - its (mild) cheating, after all.
The comparative rarity of specific monsters and a character's upbringing's chances of acquriring knowledge of some rare or regional type of opposition have all been mentioned as limiting factors for controllin knowledge skills already, haven't they ?
