By Len Deighton

Bessie/HarperCollins. 374 pp. $21.95

THERE is a wistful air about this last of Len Deighton's seven-novel series, only partly caused by Spy Sinker's forcing us to say farewell to Bernard Samson, the spy with whom the series opened, in 1983, in Berlin Game. Tough but vulnerable, cynical but childlike, extraordinary and yet so human that we feel the injuries done him by the vicious world of espionage as if they were done to us, Samson is a terrific character, and the pleasure here of seeing his troubles well-resolved is tempered by knowing there will be no more troubles to share.

Worse, we have also lost Deighton's Berlin, where the most serious of Samson's troubles occured. Berlin was as much a part of this septet as Samson, a smoky metropolis hulking under a freezing drizzle, a great city with a bloody past, divided by the Wall into the most visual and brutal of metaphors for the confrontation between East and West. Even when set in other places, Deighton's series always seemed to depend on that weird juxtaposition of a glittery, frenetic capitalist island and the glowering grey sea of socialism which surrounds it. In Deighton's novels people came to Berlin as if to a stage or an arena, a place where mortal danger gave their lives volume and vibrancy.

Now of course the wall is being sold by the chunk in K-Mart, and East Germany, which had lurked so evilly through Deighton's books, is within months of disappearing entirely, its economy proven so disastrous that even mighty West Germany may be unable to pull it from the mud, and could even end up in the mud alongside.

But if East Germany's greatest threat is only that it may prove an economic tar baby, what then of all the billions that were spent, and the thousands who died, to forestall the greater threats we had imagined? Deighton's book makes this question much more than an embarrassed awareness that history has made the traditional East-West thriller obsolete; much of Spy Sinker is rich with Deighton's conviction that both the genre and the reality from which it grew were fed on misapprehension and self-deception.

Deighton does his best to adjust his plot to new historical circumstances by arguing that the defection of Samson's wife Fiona (which set the sequence going) was a plot to destabilize East Germany, and so bring down the Wall. In honesty though, this is the weakest aspect of Spy Sinker, because Deighton is faced with reconstructing the preceding six novels to reach a conclusion which, when the series began, would have seemed far-fetched even for science fiction. Particularly because Deighton is not specific about what Fiona Samson actually does as a double agent while in East Berlin, some of Spy Sinker seems a long grasp to salvage a series which reality has cruelly and unexpectedly betrayed.

Happily, though, Deighton has never depended on plot; although his books are as engrossing as any in the business, what has always distinguished them from lesser thrillers is his mastery of setting and, even more so, of character. Here Spy Sinker shows Deighton at the top of his form, in his concentration upon the one player in this series whose story is not yet told, Fiona Samson. By itself this ends the series with a pleasant symmetry; opening in Berlin Game with what betrayal does to Samson, Deighton is able now to close in Spy Sinker by exploring what that betrayal costs the betrayer, a woman who for higher loyalties leaves husband, home, and country, to incur ever more betrayals in a cycle which may, in the end, destroy her.

However, there is considerably more to Deighton's exploration of Fiona Samson than simple symmetry; through her he is able to pick at the tangle of responsibilities to self, career, family and country that lead Fiona Samson to her dangerous defection. In each arena Deighton finds misunderstanding and confusion, as each of his characters moves forward, imagining he understands everything, only later to learn he knew nothing.

It is from this that Spy Sinker's wistfulness grows, that all of the characters have given so much, and have received so little in return. Indeed, for all that the Wall did come down, Spy Sinker questions wearily whether the price that the Samsons, and other spies, paid for that success was worth it. So many resources, and people of such talent, engaged in an activity that Deighton shows ultimately to have had so little clear purpose -- this is the stuff of tragedy, and of comedy too, which is why Deighton's series has always seemed larger and more serious than the simple entertainment of thrillers. The questions Len Deighton has raised in these novels are those we must answer in the future -- which leaves us with the only thing to be regretted about Spy Sinker, that it is the last in this remarkable series.

Anthony Olcott, author of "Murder at the Red October" and "May Day in Magadan," is translating and editing a new series of Russian mystery novels.