Since most of us at one time or another, if not continuously, keep a diary, this book will have wide appeal. Much more than an anthology of pages from published diaries, it is a nicely written account of, as it says in the subtitle , people and their diaries. Mallon has analyzed the people of the title into seven unambiguous types, and the diary writers among us will want to know where they fit in.
One kind of diary antedates the diary proper. The commonplace book is really a blank book used to record things worth recording. Ben Jonson's ''Discoveries, '' now considered to be one of the first works of literary criticism in English, is so seamlessly written that it is often taken to be personal effusions, when in fact it is almost all translated and quoted from other writers.
The diarist, on the other hand, must write his own diary. Furthermore, he must not think about anyone reading it. Mr. Mallon opens his introduction by confessing (the Confessor is one type of diarist) that ''there are about thirty of them now'' - thirty volumes of his own diary. But he further confesses that, though he has counted them in a rough way, he has not read them, at least not systematically. The diarist writes to write, not to be read.
The diary is a modern form of self-revelation. We reveal ourselves to ourselves in our diaries. Recent students of writing stress the reflexive aspect of all writing, but diarists don't need scholarship to be embarrassed by what they have written.
Mallon's categories, as if following the logic implied by this analysis of writing, progress from objective to subjective, chroniclers to prisoners. Virginia Woolf on the one hand; on the other, Anne Frank.
Mallon writes: ''To be human is to feel guilty.'' Funny, moving, unfailingly interesting, Mallon moves away from Pepys and Boswell, whom anybody I call friend has fellow-feeling for, to the world of ''Go Ask Alice,'' the horrifying confessional diary of a '70s teen-ager which a Long Island School Board sued to have removed from the school library. They lost the case but taught us all something about innocence.
The logic of Mallon's book is irreversible. Given the premise that the individual should find his own case interesting in itself, the diary, as a form of experience, will end in a cul-de-sac. As some of us failed diarists have discovered to our chagrin, it takes art to write a decent diary. Bored with ourselves, we turn to snatches of conversation we overhear, or, as Athol Fugard reveals in his Notebooks for July of 1970, to marine biology.