A provisional, three-part report on the hostage crisis--whose second, meatiest part consists of the contents of the special, May 17 issue of the New York Times Magazine. But what's of most immediate interest is the first part: an account of the embassy takeover and the hostages' 444-day captivity pieced together from interviews with 20 and what others have made public elsewhere. It does give a general sense of how the hostages fared: the first grim weeks when most were bound and blindfolded, the harrowing rash of mock executions and such, the depressing spell in underground ""Mushroom Inn,"" a relatively comfortable stretch in pleasanter quarters (conversation permitted, mail sometimes delivered)--until, after the aborted rescue attempt, most were scattered outside Teheran. It is also clear that the treatment of the hostages varied--with hostiles penalized and suspected CIA agents getting a goingover. But much of the variation was merely erratic: ""It was like they had read this is the way hostages are treated in Solzhenitsyn, treat them in random ways,"" observes Harvard-trained political officer John Lambert. That, however, is one of the few genuine observations here. The handful of revelations, moreover, is either undramatic (a few futile escape attempts) or unsubstantiated (two alleged suicide attempts). Obviously, a number of hostages aren't talking yet, or aren't talking for the record. Given these handicaps, the book's one full-bodied (if brief) episode involves Marine guard Gallego's first, captor-controlled, propagandizing broadcast--over much-criticized NBC. (There is also mention--and defense--of other hostage statements.) Otherwise, we hear that some hostages welcomed the clergymen's Christmas and Easter visits, while some didn't; most approved of the rescue attempt, but not ail; and so on. But for the moment, this is as much as we can learn anywhere. Add in the noteworthy second section (stocked with disclosures on the Shah's admission to the US and his medical treatment, on the militants' conduct, the US response, etc.) and the reference-worthy third section (capsule bios of the hostages, a chronology of events), and the book is a momentary must, whatever the scrappiness of the captivity-account.