Dickens in brief - for people who've found him unreadable.; The Portable Dickens, edited by Angus Wilson. New York: Viking Portable Library. 772 pp. $6.95 (paperback).

This knowing selection of excerpts from one of the most beloved of all writers may be confidently recommended to people who think they can't read Dickens. No doubt it was, strictly speaking, unnecessary to reprint ''Great Expectations'' in its entirety; still, it's the most digestible and delightful of Dickens's great novels - and, arguably, the best of all possible introductions to his work. Following it, this volume presents some 200 pages worth of ''Excerpts Illustrating Aspects of Dickens' Work,'' arranged by theme.

''Humor'' unwisely reprints ''The Veneerings' Dinner Party'' from ''Our Mutual Friend'': This passage exemplifies Dickens's comedy at its most self-conscious and heavyhanded. But Sam Weller's story (from ''The Pickwick Papers'') of ''the man as killed hisself on principle'' is an incomparable piece of good-natured grotesquerie. The ''Monologues and Duologues'' reacquaint us with Mrs. Sara Gamp of ''Martin Chuzzlewit'' and the Micawbers of ''David Copperfield.''

And Wilson has taken care to show us virtually every aspect of Dickens's larger-than-life sensibility. The atmospheric, sparsely written ''Night Walks'' (from ''The Uncommercial Traveller'') effectively represents his many clear and sharp picturings of London.

That city's imperfect institutions are unforgettably portrayed in satirical peeks at Parliament and the Court of Chancery (''Bleak House''), and the hilariously appalling educational abuse practiced by schoolmaster Thomas Gradgrind (''Hard Times'').

Dickens's social conscience is further seen in his ''Plea for a Hospital for Sick Children'' (a speech delivered before Parliament). His sentimentality over-abounds in excerpts from the ''Christmas Stories'' and in the semicloying ''Letter to a Child.''

Wilson has chosen these and other brief passages skillfully, but the fact remains that Dickens is a master of extended rhetoric and detailed portrayal, whose work is most effective over a long haul that involves the reader and works away doggedly at his emotions. It may be, therefore, that the lasting value of this ''portable'' derives from its superb introduction, in which Wilson (a devoted, expert Dickensian) gracefully and economically surveys the oeuvre, and also shows - more persuasively than any previous critic has - how ''the enduring traumas of his youth'' influenced Dickens's passionate humanitarianism and shaped his view of human character. There are, appended, a token bibliography and a fine, full chronology.