These are books five, six and seven in the Cormoran Strike series by Robert Galbraith.* These books just keep getting better. And longer!
I won't get into the plots here, but I will say if you are looking for a series that is set in London, has a curmudgeonly protagonist who is brilliant and troubled, and who runs a Private Investigator firm to help people solve problems/murders/brainwashing/online bullying/infidelity etc, and whose staff includes a smoke stack secretary who tells it like it is, a Scottish jokester who keeps everyone in stitches (not literally), a partner who is switched on, brave and unlucky in love, and clients who are decidedly, well, weird, this series is for you. So well written, complex, yet full of heart. Cormoran is a vet, a former police officer, and the illegitimate son of a famous rock star who never claimed him (until Corm got famous too). Family dynamics here are dysfunctional at best, but that only adds to Strike's charm. His partner Robin literally blows in on the wind, and they become best friends. Read what you will into that relationship. Slow burn indeed.
The series starts with The Cuckoo's Calling. I do think it is best to read them in order. The Ink Black Heart was definitely the hardest to follow; I started reading and listening to the audiobook in tandem to help me get through it. I love the audio versions here, the actor is SO good that the new TV show hired him to be in the show (called Strike, on streaming services like Prime and Hulu). I love this series and anxiously await the next time I can jump into Corm and Robin's lives!!
* If you don't know, Galbraith is a pseudonym for another famous writer, JK Rowling. No magic in these books, except the magic of reading a great story with great characters. Magic does exist in these pages!! :-)