That's tough. The first book that comes to mind for me is The Secret Art of Dr. Seuss. But it's entirely because of the artwork. I can't think of the best looking book off the top of my head just from a craftsmanship perspective. Maybe Six of Crows and its sequel? I actually haven't read them yet, but the editions I found looked so nice, I picked them up.

I agree with OP, if you consider the popularity of the games and show and other spinoff media, by comparison, the books are underrated.

But I think, without comparing them, the books themselves are popular in comparison to books that didn't receive any spinoff media.

Like, I really like The Tapestry series by Henry Neff, and I'd consider it underrated, and. It's definitely less read known than The Witcher.