Cast a Deadly Spell (1991)

In 1948 Los Angeles, everyone uses magic- everyone except hard-boiled private detective H. Phillip Lovecraft, who refuses for "personal reasons." Lovecraft is hired by a mysterious rich man to recover a stolen book, the Necronomicon. Investigating, he finds that the book holds the key to taking over the world by magical means, releasing the "Old Ones". (Contains several in-jokes to H.P. Lovecraft's horror fiction and also the hard-boiled detective genre.)