I like high production quality, useful manuals.
I don't want a flimsy paper cover that is barely thicker than the paper used for the interior.
But I also don't want it padded with useless filler to impress diehard fans with the huge page count. Case in point: Baldur's Gate II. Not only was the manual poorly organized, it also included reams of material that were unnecessary to have in print, such as 100 pages of spells.
The descriptions of all of the spells are already in the game itself. When you open your character's spellbook or look at a scroll, the spell's descriptions and parameters are already right there. Maybe include a few spells that you might start with, but as far as I'm concerned, this is wasted space. Civilization IV (and previous ones too) is guilty of this as well. Most of the contents of the manual are viewable in the Civilopedia, which is accessible almost anywhere within the game. On top of that, a lot of what is in the manual changed between press time and when the game was finished. And then a lot of it changed again in subsequent patches, rendering much of the manual useless.
I prefer all of the technical details relating to installing and troubleshooting the game to be printed on an insert card instead of in the manual. I rarely need to read it anyway, and certainly not more than once.
Generally, if the information is easily available within the game via its online help or a mouse-over tool tip and isn't really applicable to the first couple hours of gameplay, it doesn't need to be in the manual. Sure, include all of the options available to you during character creation, but it doesn't need to list all of the spells, abilities, weapons, etc that won't be available to you until much later in the game. And I realize I don't speak for everyone, but I like it when the manual doesn't reveal every spell and/or ability that I can obtain within the game. And it's all too easy to catch an accidental glimpse of it when reading about early choices.
For most games, it would suffice to have a manual that can fit into a DVD-sized case, or a double-thickness case at most. I hate the flimsy cardboard boxes that PC games come in now. They still take up more room than a DVD case, and are often just full of air and a CD or two in a cardboard sleeve. Speaking of which, there is absolutely no excuse for PC games coming on multiple CDs anymore. Any PC capable of running a game released in the past couple of years is going to have a DVD drive anyway.
I hated it when PC games switched to coming in the small boxes, and lamented the loss of the cool boxes of old. But those old boxes took up a LOT of room, and a lot of them were flimsy anyway. Gabriel Knight: Sins of the Fathers has perhaps the worst box ever made with its weird slanted Z shape.
I do miss the old two-piece Origin boxes from the early 90s; it was always like opening a treasure chest. They were stuffed with a stack of 6 to 8 foppy disks, a quick reference card, an installation guide, 1 or 2 game manuals, a catalog, a map or two, perhaps a trinket... Either give me a package like that (well, minus the floppy disks), or pack it in a DVD-sized case that will fit on the shelf next to my Xbox, Gamecube, and PS2 games.
And I still can't fit that goddam Ultima IX Dragon Edition box on any of my shelves. The game stunk, but it had two gorgeous manuals with faux leather covers, a set of virtue cards, an ankh trinket, a cloth map, a soundtrack CD, and the Ultima Collection CD.
I think these deluxe edition packages might be viable for the RPG market, as RPG fans seem to care the most about stuff like packaging and manuals. It always bothers me to look through pre-owned games and see just a CD, DVD, or cartridge, the packaging and manual long since thrown away (those Sega Genesis plastic cases were great; why would anybody toss those?). The two Lunar remakes for the Playstation were only sold in deluxe "collectors'" packaging here (I think Silver Star Story was $90 or $100 CAD), and they sold out quickly and are difficult and expensive to obtain now.