Two of my favorite RPG designs, in terms of usability, are The Monstrous Compendium for AD&D2 and the Imperial Lunar Handbook. The thing I love about both is they were designed for at table usability. They did this by being designed so everything you needed to use a give topic could be isolated.
The Monstrous Compendium put each monster on its own page. Plus, those pages were in a loose leaf binder. I could pull out the monsters in a given dungeon or region, put them in a folder, and then take just that folder to the game. I could even photocopy them and leave the originals at home. Even if I had 40-50 monsters in a given dungeon finding the sheet for one was a lot easier than any of the three AD&D volumes, the three volumes of All The World’s Monsters, or any other reference.
The Imperial Lunar Handbook provides information on regions of the Lunar Empire in Glorantha for Heroquest. A large section of the book provides character creation information for each region. In each case the information is setup as a two page spread. A region begins on the left page and ends on the right. To create a character for any region you simply lay the book flat on the table (it is saddle stitched and thus does so easily) open to that region.
In this age of PDF there is a lot to learn here. When formatting PDFs I encourage publishers to try to work on the one thing = one page/spread principle. You’re not facing printing costs much of the time so that is a lesser cost. Instead think of utility.
If you’re writing a monster book, for example, put everything on that one page. If the stat blocks are simple you can additional at the table tools on the bottom. You put a chart for the maximum normally occurring that the DM could fill out. When he designs an encounter he could print the page, fill in the blanks and have one page that he could use to run the whole thing. This could really well for humanoids if they have things like tribal characters (for every X orcs you have a shaman) that was common in early D&D and AD&D. If you’re doing a few monsters with high end art, don’t have it in the corner but do a full page facing and the monster stats trailing creates an effective layout.
For detailing regions or playable races in a setting the two page layout is excellent. On the facing page put the small text section explaining the region or race. On the trailing put all the tables needed for creation such as odd equipment, names, and the excellent Devil in the Details style tables. Then, a GM can just print out that tables page and hand it to the player who says, “I want to play an elf”.
The PDF era makes some traditional things (intensive art and interesting background) less useful. However, it also adds new and exciting options. It especially adds ways to create things more easily used at a table. Please try some when designing your next product.