The Tex Murphy games seem to have two main influences: The first being the American "gumshoe" crime novels of the thirties and fourties; the second, the film Blade Runner.
As for the first, I'd recommend the novels of Raymond Chandler, Dashiell Hammet, and Mickey Spillane.
Hammett created pretty much the mold for all the hard-boiled detectives that followed, the character Sam Spade. Surprisingly, though, this character was only in one novel, The Maltese Falcon. Nearly everyone has heard of this, I'm sure, mainly do to the John Huston film, but the original novel is very enjoyable and well-written.
Chandler created the character of Philip Marlowe (which is greatly influenced by Sam Spade). His best novels are The Big Sleep, Farewell My Lovely, The Lady in the Lake, and The Long Goodbye. There are some others too. They are all entertaining and very well-written.
Whereas Chandler and Hammet's books can contend with serious literature, Mickey Spillane's straddle a line and can be pretty trashy at times. They really aren't that well-written, but they can be entertaining. His main detective character is Mike Hammer and most books you find from him will probably be Mike Hammer mysteries. (I do recommend the film version of his book Kiss Me, Deadly. It's a lot of fun, very stylish, and has a plot element that's "quoted" (some might say stolen) in Quentin Tarantino's Pulp Fiction.)
Concerning the Blade Runner influence, your best bet is the writer of the original novel himself, Philip K. Dick. The novel the film is based on is vastly different, but it's a wonderfully imaginative novel. The main character isn't quite a hard-boiled detective like the others mentioned above and, of course, Tex Murphy, but he is a detective of sorts who investigates a crime. Some of Dick's other novels also follow detective characters like this. At the moment, the only one I can think of is the great Flow My Tears, The Policeman Said, but there are others. A web search should find you some if you are interested.
I hope this helps!