A Look at Tie-ins and Shared World writing
If there's anything that one can't escape when reading fantasy and science-fiction, it's commercial tie-in books wherein writers indulge (or start their career) in some shared world writing.
I grew up reading a lot of tie-in genre books. These included a deluge of gaming tie-in fantasy books like then-TSR's Dragonlance (as graced by the rising powerhouse duo of Margaret Weis and Tracy Hickman) as well as Forgotten Realms (with familiar names like R.A. Salvatore, Douglas Niles, Ed Greenwood and Elaine Cunningham).
But what really drew me to tie-in books were the Battletech books published by FASA. At that time, a little-known writer named Michael Stackpole was writing what I thought was the definitive trilogy on this universe called The Warrior Trilogy. I immediately became a fan of Stackpole such that I bought his succeeding books (The Blood of Kerensky Trilogy) and even followed him to his Star Wars novels (which later led me to try other writers in the Star Wars universe novels).
I suppose it was there that I realized I really needed to refocus my reading. That's one weakness of tie-ins/shared worlds: being the completist that I was, I realized that not all writers in a shared universe (whether Dragonlance, Forgotten Realms, Star Wars or Battletech) are created equal. Thus, even though I had favorite authors (Weis, Hickman, Stackpole), my need to read the characters' further adventures only led to disappointment as the other authors weren't able to fulfill my expectations.
Unfortunately, I also realized that you can't fault your favorite writer if they want to expand their wings and try other universes and other characters. Thus, chances are you may or may not get to read your favorite protagonists again once the story ends. Live with it.
Still, I do see the advantages of reading tie-in books. Thanks to the inherent nature of tie-in books and the shared world, these writers are able to write stories that are set in already well-thought and well-formed universes. Throw in some strong writers to write the flagship novels and you get a rousing start that readers can get into.
This is probably why-- in my search for something to get my teeth into in the shared universe of Warhammer 40,000-- I picked Dan Abnett's Gaunt's Ghosts series as published by Black Library. I had already heard some good words about Abnett's work in Warhammer 40K and I thought that I would enjoy these SF military/warfare books (reminiscent of Glen Cook's Black Company series and Steve Erikson's Malazan Book of the Fallen books).
In the end, I suppose that one way these tie-in books have influenced my genre reading, it's that you can't short-shrift the creation of the world of your story. Likewise, good writing will always tell-- especially if it's in the same world everyone is writing about.