Hard to answer, really.
The Soltis book (mine is in the old notation format...PQ4) is much older and the newest games are from the early 1970's. Some examples are from 1800's, and that's all great. Did you learn something from the Steinitz and Pillsbury classics? The "structure" of the sections is a little hard (imo) to keep track (unlike an outline). One thing this book forced me to do was break out a real board and move the pieces to follow along.
Chess Structures, being 40 years or so newer, the common strategic plans are laid out in the beginning of each section (for both sides). He does "classify" structures differently than I've seen before. One example is Najdorf. Soltis doesn't have this name, but the Boleslavsky Hole is what I like to keep calling it.
One thing I don't recall being covered in the newer book is the Closed Sicilian structures. He does cover briefly Benko and Ruy Lopez and some others.
The newer book uses the vast majority of games from the 21 century. I know one was from 1969 (I look over both) and I didn't see it in Soltis' book (Maroczy structure). I prefer Soltis' approach to naming...again, like Maroczy-Dragon or Maroczy-Scheveningen.
Rios' book has many more board diagrams for visual aid.
You asked about puzzles in another post. The Rios book has exercises at the end.
Has anyone read both of these books? I just finished Pawn Structure Chess. I'm planning on watching the videos on here that go along with it.
Before I move on, is there anything I should focus on that's not covered in the Chess Structures book?
Thanks.