HOUSES WITHOUT DOORS by Peter Straub (E.P. Dutton: $19.95; 356 pp.). Readers familiar with Peter Straub's novels, notably "Ghost Story," "Koko" and "Mystery," won't be fooled for a minute by his latest offering: "Houses Without Doors," a collection of two novellas and five short stories. They know full well that with Straub, white may very well turn out to be black, today may prove to be yesterday (or tomorrow), and what is could very well be maybe . This is an author who can plop a seemingly conventional character down in the middle of a seemingly familiar setting and within three pages all hell has broken loose: Nobody is conventional and nothing is familiar. Be warned that Straub isn't the easiest read in the world and that, at times, he can become convoluted and obscure to the point of complete bafflement. Bear with him at such moments.