Move over Roger Ackroyd. Which is to say that, long about the middle of this book you just might start to wonder, as I did, whether or not you really care who killed the Robins family. You see, this is not the "taut, masterfully executed novel" that the jacket says it will be. But so what? When you remind yourself that a reward of $10,000 awaits the reader who can solve the series of crimes that the book contains, you'll undoubtedly, as I did, slog on.

This book, masterminded by Bill Adler and written by Thomas Chastain, has a contest backing its sale: Readers are invited to come up with not just the who, but the where, when, how and why for eight unlucky Robinses. The authors' solution has already been locked in a vault. In fact, the photo of the pair on the back flap was taken in front of this repository. The vault will be opened and said solution will be announced on May 28, 1984. (Unless, of course, all of this is intended to lead to the high drama of: "Sir! The envelope, it's missing and those men from Price Waterhouse, sir, they're . . . dead.")

In any case, this is obviously more gimmick than book, but it's a good gimmick. I'm even tempted to predict that "Who Killed the Robins Family?" will go on to replace the Parker Brothers' parlor game, "Clue," as the gag gift for mystery buffs at Christmastime. This coming Christmas, anyway. There is, after all, an April 1984 deadline for entries.

This last makes me wonder why Morrow chose a summer, albeit late summer, release for this book. Contest hopefuls will want to read with paper and pen at the ready, so a beach book this is not. Maybe Morrow is hoping that it'll be all the back-to-school rage, and maybe Morrow is right. It's a more civilized, even scholarly (by comparison), form of "Assassin," the fad that swept campuses not too many semesters ago.

The trick to enjoying "Who Killed the Robins Family?" is not to try to read it as if it's a bona fide mystery novel. And the best way to do this is not to read it alone. You can split the reward. (Is that the stuff of the sequel? A sort of "Treasure of the Sierra Madre" that will take place entirely in a Mount Rainier bungalow?)

But back to the book, where members of the Robins family, and sometimes even outsiders, are bumped off at an alarming rate. Two to a chapter seems the norm. And the dastardly deeds occur in all the settings you'd expect: a locked stateroom on board the family yacht, the belltower of an eerie island mansion, a deserted London dockyard. Why, there's even a murder in one of the sleeping cars on the Orient Express (except that, in the true tradition of the genre, the authors drop some esoterica, telling us what the train is called today)!

I keep wondering how much of a solution we're going to get. (For a quarter and a self-addressed stamped envelope, it'll be mailed to you after the winner has been announced.) For instance, we're told that the reader "who catches echoes of, and references to, some past fictional murder mysteries will have an advantage." Will these allusions be pointed out? Or are we doomed to wonder if indeed we've been as clever as we thought? Will we find out how close or how far we've come?

It also remains to be seen whether or not the solution is fair. If it isn't, Morrow (not to mention Adler and Chastain) will find out the mystery fans are a vocal lot. Early on, for instance, we are told to look for more than one murderer: "For those seeking a single thread which would connect the bizarre series of murders of the eight members of the Robins family, there is none." Now, should it turn out that Tyler--the patriarch and first victim--is really alive and the perpetrator of the other crimes . . . well, maybe that's what the sequel will be all about: a writer, a promoter, and various execs at a publishing house, picked off one by one by angry book buyers.

Ah, but if the clues really do turn out to have been there, this book will be judged a hit by those of us who yearn to test our knowledge and our skill; those of us who just don't think there's any fun in being selected at random by a computer, even if it does mean that we win a dream house and $1,000 a month for life; those of us who would, if we could, watch reruns of College Bowl. You know us: We're the ones who never need to see the last scene of the Charlie Chan movie. We just stick around to gloat.