IT HAS BEEN quite a long while since readers, as opposed to cinemagoers, heard from Alex Garland. His last full-length book, The Tesseract, was five years ago. Has he been in a coma all this time? Well, yes, in a way, he has.The plots and atmospheres of both Garland's previous books were steeped in the experience of hallucination (its menace and disorientation) - in the sensation of the waking dream. A sinister imaginary friend, Mister Duck, accompanies the hero of The Beach into insanity. A dreamlike black dog haunts the suburbs of