Books

100% even for people who would consider themselves more of a casual or infrequent reader. Anecdotally, I've found it harder to sell the show to some people after they watched the first two or three episodes compared to the books for any acquaintance I know who's into Sci-Fi/reading. The descriptions of the social/society explanations are just as well written as the series' tech, and the co-authors do a great job. At least one of them was one of GRRM's editors, so the style structure is somewhat similar, but the writing is an easier read. It is written in character chapters, so as long as that isn't a total turnoff I'd recommend them.

There are some slight differences that are typical between the show/book. I believe the first 2 seasons are mostly from book 1 and then it picks up from there. I believe it's the second book, where there is a bit more of the geopolitical storyline compared to the others. Like with most reading, you get better insight to characters' thought processes and motivations. Books 7-9 aren't touched on at all in the show, and take place after the show's end. That's all I really know about the final 1/3 of the series.