The Book Thread

[The Glyphstone]

Reading them in order will introduce you to various characters and establish key elements of the setting that grow in importance later. So its not required but can be helpful - Butcher likes recurring characters and call-backs.

Death Masks is book 5 - the 1st free (Storm Front, Fool Moon, Grave Peril) are generally considered to be rather clichéd story-wise and primarily worth reading for the worldbuilding.