JUNE THOMSON offers another genteel murder for a pleasant evening of reading for the mystery fan. It's Death Cap (Doubleday Crime Club, $6.95), and again we have those poisonous mushrooms that have served up many a murder victim in the past. This time she is an attractive, comfortably-situated widow in the quiet English village of Abbots Stacey, with the vicarage, footpaths, charlady, and hidden secrets and nourished hatreds from the past. Thomson handles it very well indeed, and her Detective Inspector Rudd is a nice, intelligent man with whom to spend an investigation.

Robert Barnard gives a glimpse of Australian academic life as a rough, lazy policeman bullies his way to a solution amid scholars all the more proper because they are so undistinguished. Death of an Old Goat (Walker, $6.95) refers to the murder of a visiting lecturer from England, who still delivers the same lecture on Mrs. Gaskell that he wrote in 1922. He isn't murdered for that -- which would be justifiable homicide -- but for a long-ago indiscretion from Oxford college days. There are some diverting moments but not that many.

Filmi, Filmi Inspector Ghote (Doubleday, $5.95) takes the diffident Bombay detective to the strange, glamorous world of the Indian film industry. H. R. F. Keating's humble but smart detective is called in when the popular player of villains meets a villanous end. Ghote, besotted with his role in the glittering circle of film stars, does manage to keep his head long enough to solve the murder.