Popular 1970s TV series The Sweeney spawned nine paperback tie in novels (all original stories) plus a novelisation of the the second film spin-off.
Note the incorrect spelling on the cover of the first book (corrected in later editions). The first novel was originally printed as Regan earlier in 1975 (the title of the original Sweeney pilot) and it still referred to under that title inside.
While the first novel is credited to series creator Ian Kennedy Martin, the rest are by 'Joe Balham', who may be a pseudonym of crime writer Alan White. These all appeared between 1975 and 1978.