This sounds like it might by the Lord Darcy books by Randall Garrett. Lord Darcy was an investigator in the service of Richard, the Duke of Normandy (brother to King John). His assistant was a forensic sorcerer.
Randall Garrett wrote two books of short stories, "Murder and Magic", and "Lord Darcy Investigates", and one novel "Too Many Magicians". Another author, Michael Kurland, later wrote two more Lord Darcy novels "A Study in Sorcery" and "Ten Little Wizards".
I remember the omnibus collection you mentioned, but I can't recall the title. I know I have some of the Lord Darcy books, so maybe I have that one too. My books are an absolute mess right now, but I'll have a look - but not until after the hockey game tomorrow.
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Nid yw'r hoff o lyfr yn fyr o gyfaill.