I remember King's Quest 3 had a bunch of spells and/or potions you needed to use to win the game that required precisely worded commands, and they were only included in the physical manual.
The manual had an extensive "encyclopedic" section that was part background material, part hint book, and part ingenious copy protection. For example it included the following 'poem' as background material about one of the game areas:
Three roses laid upon the bower,
A scythe for he who cuts the flower,
A crown, a dove, most noble race!
Thy bones make sacred this dread place.
http://i.ytimg.com/vi/WUw771rvxhc/hqdefault.jpg
There were several solutions that were cryptically hidden in the manual like this, such that people who hadn't read it would get stuck at various points.
The real genius however was that this was before the Internet, and so if you wanted hints you had to call Sierra on their special 900 (toll) number to get help.
What a joy to play that game. One of the first games (I'd played) to take on a cinematic feel all in-engine.
Regarding KQ3, I miss those little hint books you could buy for the game that came with the red reading window. Totally not necessary today, but it was a fun thing to have back then.
I had a printout of the spell book, and there was a typo in it. Since it was an uncommon word, I didn't catch it, and didn't think to look it up. In class, we encountered "soporific", and I thought, "oops, I guess 'soponific' isn't right..."