Dude, it's called licensing. One creator approaches the holder of any intellectual property and says 'we'd like to take your IP and make product X with it.' The Licensing department then checks wheter or not that is in the companies interest. For example: Harry Potter Lego sets are something that benefits the Harry Potter IP. It is a huge money maker. Officially licensed Hermione Granger Sexdolls on the other hand would probably get you a lot of bad press. So these are declined.
The creator and IP holder come to an agreement. Let's stay with Harry Potter and Lego. Warner Bros. allows Lego to use the HP trademark and IP to make a
Lego Hogwarts set. Now the fine people at Lego start bricking around. Warner Bros does nothing at all, aside from sending them some digital files for promotion, Logos, etc. Once the set is done, it's thrown at the market, and people buy it. Warner Bros get's a share of the profit. So, if you buy a Lego Hogwarts set, some of the cash you spend goes to Warner Bros, some of it to Lego, some to the store you bought it at, and everybody wins.
Livensing IPs is incredibly profitable, you'd be stupid not to do it. This book is going to make Blizzard money, and at best it will cost them a few hours of an intern mailing Chelsey artwork.
Can't believe I have to spell this out for your.