I strongly recommend My Sister Milly by Gemma Dowler.
My instant thought was that a book on a murder victim by her sister would be drenched in sentimental treacle. Not a bit of it - it hits hard, and both the police and the judicial system get a (justified) drubbing.
Two excellent books.
My Sister Milly by Gemma Dowler
The case needs no introduction, and this book could easily have been a disaster - sentimental treacle. It is not; in fact, it is one of the very few true crime books I have found to be worth reading. Milly's sister gives a unsentimental and actually hard-hitting account of the murder, its aftermath and the cost on the family; the various police and defence tactics get a particular drubbing. The very high Amazon rating (4.9 from over 350 ratings) is justified.
Catching the Bullseye Killer by Steve Wilkins
This describes the John Cooper case (Pembrokeshire Coastal Path murders). The writing is artless - no flourishes or cod psychology - which is just right because the author simply puts across the quite astonishing investigation which trapped Cooper. (One of the most crucial pieces of evidence was taken from a recording of Bullseye in which Cooper participated).