Player-Viewable GM Content is a pretty handy consideration for me now that I'm thinking again about whether I want to have a core rulebook and a Game Warden's Guide. The trouble with splitting the two too thoroughly is that a person may purchase the core book without thinking to purchase the other and that should be okay. The trouble with including them both in the core book is that players often do read ahead, especially if they are the ones who discovered the system and setting. Heck, some players are notorious system collectors who purchase anything in their favourite genre to read greedily. If they like it enough, they may even convince their current GM to give it a spin. In that case, though, they'll normally stop short of reading any pre-generated adventures and may cast a less intensive eye on the monsters but everything else is fair grame.
There's a number of ways that Game Designers are now coping with this habit, especially as Game Designers generally would like players to purchase a copy because it lines our pockets and provides a wonderful financial excuse to keep producing content (notice I say 'excuse', because it's generally not something you can rely on to pay the bills so most folks do it because they're driven to do it).
Some Game Designers throw it all into the same book. This is appropriate when it really doesn't matter what the players read, when the players are assumed to have experienced the setting elsewhere or when the characters should have a firm understanding of the setting. I particularly noticed that in books based on other properties like Firefly tend to put it all in one. They may then release 'specialty' GM-focused books which target secret areas which players should not immediately know about.
Some Game Designers only put the system in the one book and everything else can be found elsewhere. Even Pathfinder does this -- likely because it's already a mammoth weighty weapon with all the rules packed between its firm covers. If the players only read the core rulebook they'd be likely to pick up on certain cursed weapons, magic item uses, and guess at an NPC's class based off their spells but even in those case they'd need an impressive memory to do so. The rest of the information is packed into 'specialty' books which are divided between Player Friendly and GM only.
Some Game Designers create a core rulebook and then create a player friendly book. Normally both of these contain the rules which is a bit of a repetitive dump of information. The advantage of this is that players normally don't get a lot of advice and have to read the GM advice sections (which is normally, but not always, reasonably player friendly) and then apply the advice backwards to figure out what to do. The disadvantage of this is that normally the same content could benefit both. Normally what actually happens is that the Player's Guide is a cut down version of the core rulebook. Stats, some tips and some setting without any monstrous goodness.
The other method is to create a core rulebook and then a GM focused book. The GM book has the monsters, secret setting, rules and other special sundry items inside of it. Normally what this boils down to is that the core rulebook has a little bit of everything but the GM book goes into things a lot more deeply. Oftentimes the GM's book suffers from a bit of bloat, either because some information is still being withheld so the Game Designers can sell more 'specialty books' or because the writers haven't learned the art of succinct yet evocative detail-oriented text. In other words, there's a lot of fluff and not much delivery. This doesn't mean it's not a good read, you just don't get as much actual advice and information out of it. (NB: Too much brevity can actually cloud learning as the brain sometimes needs the same detail put three different ways to soak it in and because there needs to be breathing space between each major point which can be assisted by expanding on a single point for at least a paragraph).
I'm not sure which direction I will go with mine. I know that I will include some Player-Viewable GM content even if I create a single core rulebook or a focused GM's book because, well, some secrets don't need to be secret. Hiding information on how to create a crime scene with forensic detail also means hiding information on how to analyse one, after all, and never revealing a single potential monster also means the player has to fall back to expected methods of despatch borrowed from other games which may not actually either a) work or b) involve something a person would actually do.
In other words, drop Slenderman among a group of D&D players and they will understandingly attack it if they don't know any better. They will then die and have to create new characters. This would potentially be a good start to a horror campaign in D&D where the in-game assumptions are turned on its head but would suck as a start for a Call of Cthulhu campaign where the academics really should have fled screaming because that's what an academic would do the first time they were confronted by a bizarre otherworldly being. The main reason the D&Ders *would* and *should* attempt to attack a monster outright the first time around is because that's what they're expect in the rules of the game.
Seeing chapters on "Constructing and deconstructing crime scenes", as an example, could help kerb such behaviour as the players, through reading it, get a better idea of what they should and shouldn't be doing.
Anyway, what do you guys think of the different methods of rules and setting delivery? Prefer a Player's Guide or a GM's one to come out alongside a core rulebook (core meaning one that both can read)? Prefer it all in one?