I've always been fond of the PHB / DMG design idea. I think any game that is designed at least in part so that it is accessible to new or non-gamers should be broken down between "material that the player needs in order to make and use their character" and "everything else". This is not to say that the "DMG" needs to be a secret document only GMs should own, but rather that a player should have no need to read it in order to play the game, especially if they are a new player or a very casual gamer who isn't really interested in learning all the rules.
I decided to take my own advice when writing Commando. The game is broken down into two main documents; the Operative's Handbook and the Operations Manual. The Handbook is everything the player needs to:
A) Get a 35,000 foot overview of what the game is about.
B) Understand the basic mechanics behind the system and how to use them.
C) Create a character and prepare the character for adventures.
The Manual is, well, everything else; optional rules for characters, gameplay and combat, how to plan and create adventures that fit the sort of gameplay that Commando was created to achieve, reference materials such as books, movies, and even video games or other RPGs, and much more.
When considering my "PHB / DMG" method of design, I decided to take a look at those two books and see if I was actually making sense when I came up with my theory. Looking at the roughly 120 page PHB, I see right away that roughly half of it is spells, about 60 pages. If I cut that away, the game is about 40 pages of actual rules, roughly 3/4ths character creation and 1/4th "playing the game". The remaining 20 or so pages are appendices and reference sheets of questionable value.
Meaning that if you cut out magic spells and some of the tacked on appendices, the PHB is really just ~30 pages of how to make a character and ~10 pages of how to play the game. Thus far, I'm actually fairly on track (not in terms of page count, but rather in terms of subject matter).
Turning to the DMG, which is ~230 pages, I see that roughly 80 cover alternate, advanced, or expanded rules for characters and gameplay, then 80-ish pages dealing with campaigns and adventures, and roughly 70 pages of appendices covering various random pieces of information (including reference materials). Again, this seems to groove with what I'm creating; the Operations Manual is about one third expanded gameplay, one third adventure and campaign creation material, and one third appendices such as what will definitely be a rather large section on reference material.
There will be a third "core" document serving as a much expanded arms and equipment guide. The Handbook has very brief descriptions and statlines for a selection of common weapons, but the A/E Guide will be much more comprehensive and include vehicles as well. Not exactly a "monster manual"; all the bad guys are included in the campaign and adventure portion of the Operations Manual, so it's sort of a switcheroo. And, if you want, you could certainly consider an MG-42, 88mm Cannon, or a King Tiger tank as "monsters" if you really wanted.
So this little bit of investigation and comparison makes me feel...vindicated? I'm not sure about vindicated, but I do feel at least like I'm moving in the right direction. I've always favored the two book method of RPG design over one "big core book", half of which is unnecessary for the player. That the progression of my own two book method is following roughly in the footsteps of the original duo makes me happy.