Career of Evil by Robert Galbraith (aka JK Rowling)
Must read this master story-teller
From the Cormoran Strike novels. Another brilliantly weaved crime that poses multiple suspects and as many reason they might have as couldn't possibly have, committed the crime.
Strike has more people working against him than for him, and his business starts collapsing, but his secretary, Robin, is always there. Their relationship remains respectful and yet frustrating, but it deepens through shared horrors and triumphs. JK Rowling has a gift for developing a variety of layered and realistic characters.