This is an unusual and important book. As the sub-title records, it is ''an eye-witness account of Peter Brook''s production from first rehearsal to first night.'' The production in question was a famous one that had its opening night on 27 August 1970. Audiences loved it, as, indeed, did most critics, Clive Barnes of the New York Times going so far as to say, ''It is a magnificent production, the most important yet of the world''s most imaginative and inventive director. If Peter Brook had done nothing else but this Dream, he would have deserved a place in theatre history.'' All the more reason therefore to be grateful for this unique account of this production''s gestation during its weeks of rehearsals.

In a typically stimulating preface, David Selbourne concludes by saying, ''What follows, then, is not hindsight. It is, instead, an attempt to reinstate the true density of first impressions . . . as well as to recover fleeting moments in the posthumous life of Shakespeare''s writing, in one of the very greatest of his plays.''

Simon Trussler, in his introduction, judges it to be ''an important work, both in the critical portrait it offers of a leading director at work, and in its demonstration of the kinds of bluff, self-deception, and unhappy accident that can as much contribute to the making of a theatrical ''triumph'' as our more orthodox expectations of creative struggle, fruitful improvisation, and dialectic discovery - present in full measure though these undoubtedly were. All the ingredients are there in the seven-week mix of rehearsals of which the detailed record now follows. ''

''An indispensable theatre book . . . It explains more than we could have hoped how the miracle was wrought.'' John Barber, Daily Telegraph

''The process of theatrical creation comes across with rare force, expressed in language - Mr Selbourne writes very well - of rare beauty.'' Michael Coveney, Plays and Players