Of all the demons bedeviling a would-be critic, the most dangerous and seductive is the one that tempts said critic to judge a book not by what it is, but by what it ought to be. Presumably the author has a specific aim in mind; he or she should be judged by the success or failure of that intent. Yet the line is not always easy to draw, and "Tidewater Dynasty" may drag me over it into unfair criticism. It comes so close to succeeding in what I think it ought to be and in what it might be.

According to the publisher it is a novelized biography. The pernicious habit of categorization is particularly inappropriate here. "Tidewater Dynasty" is a novel only by the widest stretch of that definition, and it is a biography not of an individual but of a family -- the Lees of Stratford Hall, Va.

In this letter fact lies the major failure of the book, I believe. It tries to do too much. Individual members of that remarkable family have been featured in innumberable biographies and works of fiction. Its most famous son, Robert E. Lee, appears only at the end of this volume, as a small child, and in a somewhat contrived prologue and epilogue. Agonizing over the choice he must make between loyalty to country and loyalty not so much to his native state as to a compex web of other, less tangible values, Robert Recalls his roots -- the generations who made him what he is and whose history, one assumes, makes his eventual choice inevitable.

The genealogy of the Lees might serve as a record of early Virginia history. Most of the men were famous as statesmen, soldiers and politicians. They intermarried, and were friendly, with all the other prominent families of the period. Harrisons, Washingtons, Jeffersons, Carters and others pop up every few pages, affording the reader a nice little thrill of recognition.

We know most of these people and we are pleased to meed them en famille and without their wigs. By providing a framework, both physical and chronological, the authors give the informed but nonspecialist reader a sense of continuity and relationship which his other reading may not have supplied. Equally admirable are the deft summaries of political events provided by the conservations of men who shpaed and were shaped by those events.

Unquestionably, these men were a gifted lot; but how well might they have succeeded, one wonders, if they had not been born into the right class and the right sex -- halfway up the ladder that led to wealth and high office while they were still in diapers, so to speak? Roberts and Seely show us that the Lee women were as interesting as their husbands and brothers and sons, though they had to contend with a system that was only slightly less restrictive than the doctrine of kinder, kirche und kuche. None of them really fought free of the system, but several won through to a form of personal liberation. Missy, daughter of Thomas Lee, who built Stratford Hall, is a particularly delightful personality. She deserves a novel of her own.

Multiplicity of characters is not a weakness in itself. Any major historical novel has dozens of people wandering through its pages. For maximum effectiveness, however, an author does well to focus on only one or two of them, relegating the others to minor roles. The Lees of Stratford produced too many major characters, and the authors never really reconcile the sometimes conflicting demands of biography and novel. In part this may be a problem of technique.

One particulary unfortunate weakness is that of the transitions. As every writer knows, this is one of the most difficult tricks of the trade -- leading the reader gently and smoothly across gaps in the chronological sequence. The authors of "Tidewater Dynasty" use interpolations, summaries of events in the lives of the characters as recalled by Col. Robert E. Lee, brooding over the complex threads of his heritage in his dark hotel bedroom. It doesn't work too well. There is often a jarring break in mood instead of a neatly paved bridge.

"Tidewater Dynasty" contains an enormous amount of factual information, facts about people, places and events both personal and political. A reader who lacks background in Virginia colonial history may feel overwhelmed by so much date, charmingly presented though it is. However, for those who have done a little reading and want to learn more, the book has much to offer. It ought to make those trips to Williamsburg and the James River plantations much more fun.