HERE IS a splendid jeu d'esprit by John Fowles, which will give delight to anyone who is truly fond of literature, though it may cause annoyance to readers who have little sense of humor.

The book cannot be called a novel, for it has no story to tell. The title is not fully explanatory, and will baffle those to whom a mantissa is a fraction of a logarithm. But if we go to the greatest dictionary of them all, the Great Oxford, we find that a mantissa is "an addition of comparatively small importance, esp. to a literary effort or discourse"; it is interesting that the illustrative quotations this dictionary offers to show how the word has been used are all from theological works. What Fowles has given us is no trifle, though it is not physically bulky, and not theological in any ordinary sense of the word.

It reaches toward the world of the gods, however, for it is about the relationship of an author with his Muse, in this case Erato, traditionally associated with erotic poetry (and geometry, for these Muses were versatile), whom Fowles thinks most likely to be interested in the modern novel.

Erato appears in several guises, and in all of them her concern for erotic poetry, or anything simply erotic, is amply apparent. Her occasional associate, in the mind of her author, is Persephone, or Kore, who turns up in the book as Nurse Cory, a beautiful Barbadian who is simpler and more generously erotic than Erato, who occasionally appears as the minatory Dr. Delfie. As you can see it is a book of some verbal complexity, and you have to be wide awake as you read it.

It will be helpful if you examine the jacket carefully before you begin. It carries an etching by Picasso, named Sculptor and Model. A female figure, plainly a goddess, though a lesser one, is looking at herself in what may be a mirror or possibly a portrait; she wears a chaplet of flowers, appears well pleased with what she sees, and although from the waist down she is white, from the waist up she appears to be black. Gazing at her, with slightly weary, proprietorial eyes, is a large naked man, handsomely bearded. When you look at the author's photograph on the back of the jacket, you see that this mythological figure might well be an idealized portrait of John Fowles. But the third element in the Picasso is what may be a head fallen from a broken statue, or it may be another rendering of the head of the sculptor, for its gaze is intent, perplexed and by no means pleased.

As indeed the sculptor, or the author, has every reason to be in the book, for Erato, or Dr. Delfie, is as distracting, capricious, tender, critical, admiring, captious, bossy, yielding, cold, hot and in every way such a mingling of opposites that one might say there was no knowing where to have her if it were not that the author (whose name is Miles Green) seems to manage pretty capably in that respect. The relation between author and Muse is a powerfully erotic one, although that is not all there is to it, and here again Erato is tease, prude, tough expert and untouched virgin in a series of bewildering changes.

What is the book about, you are asking. It is about what I have been describing; it is about how an author -- or better say how Miles Green -- encounters and is inspired by his Muse, and it is quite the liveliest description of this fairly common encounter that I have ever read.

Muses are usually dealt with respectfully, not to say gingerly, by poets and prose writers who speak of them at all. In the 19th century there were many pictures of the Poet, seated at his desk, preparing to write something (with a quill) dictated by a female figure of asexual aspect who hovered at his shoulder. Muses were to be heard, but not touched.

Not so in Mantissa; Erato is touched, and indeed sometimes thumped, and in her turn she lands several shrewd blows on the author whose inspiration she is. Not only do they fight and make love; they talk, splendidly and entertainingly, and Miles Green's attempts to bring Erato up to date on the latest developments in the novel are hilarious, particularly those in which he explains the necessity for the thickest and most impenetrable existentialist gloom.

There is little here to give comfort to academic devotees of the latest fashions in the novel, or to the earnest proponents of Women's Lib. Erato is a partner, and sometimes senior partner, in what Miles Green writes, but she is not herself a writer. She is the Eternal Feminine, who does not need to be freed because she has never been bound, though now and then she is sat on, and for good reasons.

The book is a splendid lark, but no trifle. It is the best possible evidence of the relationship between John Fowles and his own Muse that he can spin a web like this which is so light, and yet so strong.