The origins of Osiris are surprisingly murky. His name appeared for the first time in the late Old Kingdom, and within a century he became the most important of Egypt's afterlife gods; the Pyramid Texts refer to him constantly. The mystery surrounding Osiris has spawned all kinds of speculative theories in the scholarly community, sometimes pretty far-fetched.
In this reasonable-sized but very dense book, an expansion of his 1966 work The Origins of Osiris, Griffiths examines all the aspects of Osiris's early character, looking mostly at evidence from the Old and Middle Kingdoms. He addresses seemingly every origin theory that was raised in the century before he wrote, and he delves into such academic minutiae that when I first read partway through the book I wasn't sure what he was arguing for. By the time he's done taking apart the other theories, though, his point becomes clear. Whereas Osiris was frequently thought to be a mythologized version of a real king, or a god of agriculture who was tied to death and rebirth, Griffiths shows that these aspects of his character were secondary developments. Osiris was originally a god of the dead and the underworld, perhaps one with a forbidding and ominous reputation. The king was believed to become one with Osiris upon entering the afterlife, ruling the world of the dead as he had once ruled the world of the living. The influence of this idea softened and broadened Osiris' character in the late Old and Middle Kingdoms, so that he became the mythical king and god of rebirth that we're familiar with from overviews of Egyptian religion and myth. The result is a very thorough, but sometimes very tedious, examination of the early evidence about Osiris. Because the book obviously isn't aimed at a popular audience, I value its thoroughness more than I mark it down for its tedium.
The idea that Osiris began as a god of vegetation or fertility still circulates today more widely than it should, and a few Egyptologists still propose origins for Osiris that are as dubious as the ones that Griffiths shredded. His work should have inoculated Egyptology against such speculative hypotheses. Large parts of Griffiths' own argument may not stand up to scrutiny—some recent reinterpretations of the evidence point to Busiris rather than Abydos as Osiris' more likely place of origin—but he does seem to be right that Osiris was always a funerary god.
Recent authors have added evidence that is only partly compatible with Griffiths' hypothesis, such as Harold M. Hays in "The Death of the Democratisation of the Afterlife" (in Old Kingdom, New Perspectives) and Racheli Shalomi-Hen in The Writing of Gods. The most recent and rigorous examination of Osiris' origins, in Following Osiris by Mark Smith, ends by more or less saying "we don't have enough evidence to know". I have my own vague ideas on the subject, based on the evidence pulled together by all these authors, but I wouldn't call them anything more than educated guesses. Osiris remains mysterious, but Griffiths studied the mystery more carefully than most.