When they run out of ideas, they simply expand it through an expansion pack and they added a completely new element to the game. When a game comes out and people play it for a while, the game books and strategy guides start filling the stores. People buy these to help get a better idea of the game and to make it through faster.
These games are…massive…and it’s easily to get lost for days or weeks at a time. The guides are great for beginners, but they lose their appeal quickly. If you arrived late to playing the game or an avid player, then the book will be of little help. Developers have tweaked the game and made so many additions that the directions the books give you could be wrong.
That’s not likely to happen during the initial quests, but as you go further, it is very possible. The publishers don’t get around to updating their guides that often and then just write a new guide for the expansion. Rather than buying a premade guide from a store, I suggest looking for guides online. There are many game sites that have a special section for strategy guides and walkthroughs. These are updated regularly and new ones are always being added.