A spy thriller that really happened; and an adoring portrait of Lee Iacocca

Sigmund Rosenblum, a.k.a. Sidney Reilly, may not be, as some Britons claim, ''the greatest spy in history,'' but he is certainly turning out to be the greatest spy in television history so far, bar none. And that includes fictional undercover men such as Ian Fleming's 007 as well as le Carre's Smiley. Sidney Reilly has one great advantage over all the others - he was a real person!

You can join millions of spy buffs in marveling at the incredible exploits of Mr. Reilly, played by Sam Neill, on the ''Mystery!'' series: Reilly: Ace of Spies (PBS, Thursdays, 9-10 p.m., check local listings for repeats). Since there are 12 parts to the series and only two have been aired so far, it is possible (and recommended by this critic) that you start watching next week if you are not already watching. Each individual episode can stand by itself, and the first episode is the slowest one. Some critics in Britain have called ''Reilly'' the best TV series since ''Brideshead Revisited.''

''Reilly: Ace of Spies'' is based on a book with the same title (Penguin, $3. 95) by Robin Bruce Lockhart. First published in England in 1967, the book was well received there but made little splash when Stein & Day brought it out here the same year. Now, Penguin, anticipating a big boost in sales because of the TV series, is republishing it in paperback.

As part of the big push for the book (and, incidentally, the series), Robin Bruce Lockhart is now in America telling his own story as well as the story of Reilly. He visited the Monitor New York Bureau where he felt quite at home, since Lockhart has spent 15 years in the newspaper business, eight years of it with Financial Times as foreign manager and then close to eight years with the Beaverbrook papers. ''So,'' he smiles confidently, ''you see, I am an investigator at heart.'' During World War II he was in naval intelligence for Britain: ''Liaison work with our own secret service,'' he explains.

Did he ever plan to make it a career?

''No, my father was in it and I knew it was nasty. There's no glamour to it at all. It's a filthy business.'' He says this with vehemence, although he appears to be a mild-mannered man, conservatively bespectacled and dressed in sweater and tweeds, the look of an Oxford don with just a touch of Fleet Street.

How did Lockhart first get involved with Reilly?

''My father was in Russia, sent by (British Prime Minister David) Lloyd George after the revolution as a kind of unofficial envoy to establish relations with the Bolsheviks. Eventually he got arrested for attempting to assassinate Lenin, finally being swapped for (Soviet diplomat Maxim Maximovich) Litvinov. My father, who, as you may gather, was in the secret service, knew Reilly in Russia , as did many of his connections. They were in and out of our house in Britain all the time. As a boy, I was excited by it all.''

Lockhart believes that many spy cases are more outrageous than fiction. ''The only things the public gets to hear about in the espionage world are failures. Either defectors or something somebody's missed out on. You never hear about successes for obvious reasons, although all sorts of astounding things have been done.

''In the last war, the most astounding thing was what they called 'Enigma.' We got hold of a German cipher machine and we were getting signals that Rommel was sending to Hitler before Hitler had them himself. That story has never been told properly.''

Did Lockhart ever consider using Reilly as a fictional character?

He shakes his head. ''He's so extraordinary, I don't think you could. He's so contradictory in his behavior that if you put it in a fiction story, a critic would probably say it is stupid and unbelievable.''

Is Lockhart pleased with the TV series?

''Every writer has reservations about a film version of his book, and it's difficult for me to be objective. I think the series as a whole is very good, but my only criticism is in the early episodes, because I think they're a bit slow. They stretched those early episodes, whereas in the second half of the series they stick closely to the book . . . and it moves faster.''

How did Lockhart do his research?

He laughs. ''That's a book in itself. When I wrote this book, most of Reilly's intimate colleagues were still alive. My father was alive and I had access to all his papers and diaries. George Hill, who figures prominently in this book, was Reilly's closest associate in Russia. He gave me an enormous amount of help. . . . My mother knew Reilly well, too, and I got help from a woman who used to be his mistress. And I managed to find Reilly's last wife and got much information from her.''

''Reilly'' is Lockhart's only published book so far. He helped edit the diaries of his father - Sir Robert. There were 200 volumes of diaries to sort out, which were published in two volumes. ''I've just finished another book, though - a tribute to the Carthusian monks. I've been researching it for several years. It's just about the only religious order never reformed. The book is totally nonfiction and not meant to be a best seller. I've done this for the love of it.''

How long has Reilly been dead?

''Nobody knows when he died. He ended up in 1925 in the hands of the Russian secret police. But he can't be alive now, because he'd be about 120.''

What did happen to Reilly in the end?

''He seems to have been shot by the Russians, but I have evidence that he was alive much later. I reckon that Reilly went over to Russia in the end. I've dug up Russian dissidents who know the story. There was a Russian defector named Orlov a few years ago who knew a lot . . . but the Americans treated him so badly he didn't play ball. If not for that we would have had (Russian spy ''Kim'') Philby before he went off to Russia. So we can blame America for that.

''You know, there have been lots of books about British and American defectors, but there have been no books about the Russian top cats who recruited them. That story hasn't ever been told properly, either. . . .''

The excited gleam in Lockhart's eyes indicates that there are several new nonfiction spy books in the offing. The 'soft' documentary

''NBC Reports'' is fast becoming the major repository of the soft network documentary. A few months ago, NBC anchor man Tom Brokaw narrated a Chinese documentary so soft it could have passed for underdone egg foo yong.

Now, Iacocca: An American Profile (Sunday, Jan. 29, 7-8 p.m.) is being substituted for the quirky ''First Camera'' against the hard-edged ''60 Minutes'' on CBS. It is so soft-focus that it could be mistaken for a long commercial for the political candidacy of Lee Iacocca, the Chrysler president, who has become a household name since the Chrysler TV commercials.

Correspondent Tom Brokaw spent as much time with Mr. I. as he has spent with any presidential candidate, if not more. It is an intimate and almost adulatory portrait - exploring both family and business relationships.

According to Brokaw, ''The private Lee Iacocca can be shy, sensitive, and old-fashioned . . . an old-fashioned Italian patriarch.'' But producer-director Tom Spain shows us a bit - but only a bit - of the other side of Iacocca, too - the man the unions must fight to get what they consider a fair share of the newly earned profits of Chrysler.

Undoubtedly Iacocca does represent the epitome of the American dream - the son of immigrant parents who rises to the top of the corporate heap, borrows from the government to save his company, then pays it back with interest. But we have already heard that story in great detail.

What I would like to see now is a documentary that digs a little deeper. For instance, why was Iacocca fired by Ford? What do the auto union heads really think of him? How can his $400,000 salary be justified in the face of his demands that workers tighten their belts even more?

Iacocca is treated almost reverentially in this blandly charming documentary. Producer Spain, responsible for some major hard-edge documentary triumphs in the past, seems to defer to the Iacocca created by the imagemakers determined to make Chrysler a winner. And Brokaw, playing the role of the celebrity interviewer breaking bread with the family, is not much help in any attempt to turn this puff film into an incisive portrait.