Elantris by Brandon Sanderson. More about the author than an individual book, but it was the first entry in what is now a very large universe called the Cosmere. It can arguably be pointed to as the beginning of a major change to the landscape of the fantasy genre.
Aside from the massive empire that Brandon has built over the years from his books, he also brought the concept of a hard magic system to the forefront of the fantasy genre. He even established what are called “Sanderson’s Laws” on the subject. For example, Sanderson’s First Law states “an author’s ability to solve conflict with magic is directly proportional to how well the reader understands said magic”
I believe there had been authors who had done magic systems that had specific rules and limitations before, but none achieved his level of success, nor had any really taken such an academic approach to writing and attempted to define specific concepts and writing elements. Now almost any fantasy fan can tell you what the difference is between a hard and soft magic system.
Multitudes of authors followed in his footsteps and began defining their magic systems in more detail. Sanderson has achieved international fame and built a multi-million dollar empire. Most recently, he held the single largest kickstarter campaign in history.
So yeah, I’d point to Elantris as the beginning of this.
I honestly felt the same. I finished the first book and just didn’t enjoy it at all. Couldn’t bring myself to even pick up the second.