Looking for recommendation (mystery fantasy / scifi)

I’m looking for a fantasy / sci fi book (series) that isn’t too high fantasy. Nothing too much focussed on magical systems or wizardry)

What I’m looking for is a book that has a big mystery to it. Comparable to the attack on titan manga series, where the reader learns along with the character(s) how the mystery unravels. Another example to this is the story arc north of the wall in game of thrones (white walkers) , but we all know that this had unfulfilled potential.

Thanks, looking forward reading your recommendations!

by StefDV