While considering how many existing books that games can be based on with an existing fanbase and how few authors that can get their book published as it is, you would certainly be wasting your time. Writing story for games has to be a part of the whole design process. For each game, there are many developers who learned game programming because they wanted to tell their story.
You can however forget about the big studios. You don't need them. Because you are probably an artist who want to tell your story without being limited by corporate interests. During a job interview at Electronic Arts, they asked me why I didn't just create my own company, which they saw like a big adventure compared to being in the huge office landscape doing one tiny part for years. Being independent means artistic freedom and owning your work.
If you write the book first to take the path of The Witcher and Harry Potter, count on having to write at least 10000 pages before being a professional author. After that, temporary trends and knowing the right people leads to getting the book published. Then you might earn $500 per month even if you have many fans, because the publisher takes most of the cut for all the failed books they had to invest in.
If you make the game yourself, you can become the next big game company and own it. Learn comic drawing, voice acting and a game making software. If you simply say “I'm just an author”, then that is all you can ever become. One month of practice and you will have made your first game. Then find a team of hobby developers to improve it with an open mind to changes.
Edit: Saw your reply after posting. You are one the right track as an author if you have published already.