Often the knowledge used in Private Eye novels is also on show in action thrillers. Private eye novels. Plenty to choose from, with Sue Grafton one of my current favourites. Details on surveillance, bugging, police procedure, wounds, weapons, forensics, drugs, crime rings, criminal types and behaviour, fighting techniques, violence. Authors such as Marion Zimmer Bradley, Clive Barker, Tad Williams, Robert Jordan, Peter S. Beagle.
History can add realism to fantasy in terms of grit.
A knowledge of drugs and hallucinogenics is useful if there are shamans. A knowledge of nature is useful when inventing fantasy worlds and creatures. Science Fiction and Medical thrillers. Robin Cook writes medical thrillers. Michael Crichton writes medical thrillers and books with a sci-fi bent.