There's at least two different types of 'true crime' book (in my humble layman's estimation).
Beginning with Truman Capote's 'In Cold Blood' (though he was not exactly the first, as is often claimed) then on to writers like Thomas Thompson ('Blood & Money', 'Serpentine') and then stuff like 'A Murder in Canaan' and 'The Shoemaker' (in more recent times, both authors female)...and then more famous titles like 'Fatal Vision' and 'Executioner's Song' and finally, notable works by Joseph Wambaugh ('The Onion Field', 'Echoes in the Darkness') ...by now I'm sure you know the kind of book I mean. In spite of themselves, they are written novelistically, and unfortunately at times, somewhat sensationally.
Reference & research books are a whole other matter. Very different style of book. These are the kind I would rather steer you to. There's less verbiage to wade through; and more cold, hard, data. I think for the sake of creating something unique (unpolluted by other authors) this is perhaps the best bet.
These days there are even reference books specifically written by crime experts for the crime author's markets. An example of this trend might be:
'The Crime Writer's Reference Guide: 1001 Tips On Writing the Perfect Murder' by Martin Roth
...but there are many others too. Good luck!