Bookstores are getting larger and larger. Thanks to coffee shops, cafes, even eateries inside the bookstores, one can spend a whole day inside the confines of a bookstore (longer if the store is open 24/7) nowadays. Of course, you can't just fill the extra space with just gifts, cards, and DVD's. In order to be called a bookstore, a store should have a majority of its shelves filled with books.
With a bigger store, the storeowners need to fill the shelves with more books, and you can't just fill them with just copies of The Da Vinci Code and Harry Potter (as popular as these books are, there are people who prefer to read something else). Fortunately, there is no shortage of books available — you name a topic, and there is a book about that (I'm sure that if one looks hard enough, one can find a book about alien cuisine or the sleeping habits of penguins). You don't have to be a great literary mind to put your words to print anymore — witness the number of atheletes, singers, cubicle-dwellers, and football hooligans who have put their writing on the shelves of a bookstore these days.
A growing portion of a bookstore is filled with books related to movies and video games. Gone are the days when all books related to television shows and movies are badly-written and are read only by nerds and die-hard fans of the shows (of course, they still read them). Today, many of the books written on these subjects are quite well-written and, thanks to video games' rapid growth in popularity in the last decade, their readership is much wider than before. As long as the books don't add a lot of elements with little relation to the original work, many fans will dig into their pockets to get the books based on their favourite movie/game.
The same may not be said about movies based on books. One common complaint people make about movies based on books is that the movies don't follow the book closely (if you have read King Solomon's Mines and watched the movie based on the book, you WILL wonder how the producer(s) received the green light for making the movie at all). To be fair, the storylines of many books are too long to be fitted into one movie (for example, any recent Tom Clancy novels) and the producers have to figure how to cut the length of the book so that the scirpt is short enough for a movie.
As for games based on novels (NOT movies based on books, which are often just cash cows and not much fun to play), the producers are usually given more leeway. Often the game makers can add new content (for example, in the novel the main character/group may just have a brief layover at a town, but the town may play a much bigger role in the game — for getting information, talking to the residents, doing a few errands, etc), create new ending(s) (since many people are no longer satisfied with adventure and role-playing games that have only one ending).
When people work to turn a book into a movie/game (or vice versa), they do their best to make sure that their work don't deviate much from the original. However, it would be unrealistic to expect the spin-off stays completely true to the original work.