Free theater tickets aren't the only perks of the drama reviewer's trade. One of the extracurricular pleasures consists of browsing (not drowsing!) through the reference works that cross one's desk on their way to the bookshelves. There is something vicariously satisfying about the contemplation of other people's research. So much well-ordered diligence and patient study commands respect. An occasional carp or mental question mark is par for such a reading course. It adds to enjoyment. For, as writers of reference works know full well, readers of reference works are the sidewalk superintendents of the communications business.
Judged by the volumes I have received, this has been a good year for theater reference works. Notable among the most recent is The Oxford Companion to American Theatre (Oxford University Press, $49.95), by Gerald Bordman. This 734 -page addition to the proliferating ''Oxford Companion'' library contains nearly 3,000 entries, covering every aspect of American theatrical activity, from colonial days to the present. Besides the huge cast of players and playmakers, there are summaries of theatrical movements and institutions, thumbnail histories of key Shakespearean and Shavian productions, and brief accounts of notable (and even notorious) incidents in the unfolding drama of the American stage.
Mr. Bordman acknowledges that his compilation duplicates some of the material in ''The Oxford Companion to the American Theater,'' edited by Phyllis Hartnoll. But the accent and emphasis of the new volume are American. Furthermore, by Mr. Bordman's definition, ''Theater'' covers some references to such performances as circuses, minstrelsy, vaudeville, and tent shows.
The author admits to confining his choices of contemporary dramatists and producing organizations ''to the theatrical mainstream.'' This means, for instance, that there are entries for David Mamet and David Rabe but not for Christopher Durang, A. R. Gurney Jr., Tina Howe, and Wendy Wasserstein (to mention a few of the more recently accredited arrivals on the playwriting scene). Mr. Bordman consolingly observes: ''We have no doubt that if the newcomers realize their early promise they will command entries in the future.''
The long awaited update of an invaluable reference work has occurred with the publication of The Crown Guide to the World's Great Plays (Crown, $24.95), by Joseph T. Shipley. A subtitle suggests the scope: ''From Ancient Greece to Modern Times.'' According to the dust jacket, the volume contains 750 play plots , performances, casts, analyses, and critical opinions. The project is invested with the scholarship, discernment, and relish that the veteran Mr. Shipley brings to such an undertaking.
''Since the publication of the 'Guide to Great Plays' in 1956,'' writes Mr. Shipley in his preface to the current edition, ''new types of drama have appeared. The work of the 'angry young men' has sparked upon the scene; plays without plots have rambled across the stages; the absurdists have indulged their fantasies, finding no meaning in life; dramas have striven to bypass the checks and restraints of the intellect in order to drive directly at the basic human feelings.''
If the foregoing tends to identify Mr. Shipley's position, the extensive selection of quotes in this 866-page volume represents a fair and balanced cross section of critical opinion. Mr. Shipley notes that assembling the new guide has been a process of both subtraction and addition. Even the Greeks and Romans have been culled ''to include only those that have to their credit more than the accident of survival.''
A whole generation of playwrights has emerged since the 1956 ''Guide to Great Plays.'' Not surprisingly, some 100 new plays are covered in the present volume. They include the work of Edward Albee, Samuel Beckett, Arthur Miller, Peter Shaffer, Harold Pinter, Tom Stoppard, David Storey, Alan Ayckbourn, Sam Shepard, and David Mamet. Whatever the playwright or period, Mr. Shipley's brief chronicles re-create, wherever possible, a sense of the impact of a work on its immediate audiences and the commentators of the time.
Of his own credentials and bona fides, he writes: ''I have been a professional reviewer of the New York theater since 1918, and for many years have also covered plays in Paris and London. I have thus seen many of the plays in this volume at their premieres and most of the older ones in revivals, sometimes several times. I have seen Hamlets beginning with Sarah Bernhardt. I have seen every Gilbert and Sullivan company in New York in this century. I still feel that the theater is the most rewarding form of public art.''
Theater buffs will say ''amen'' to that.
The most recent addition to the theatrical reference bookshelf has been The Best Plays of 1983-1984, edited by Otis L. Guernsey Jr. ($24.95) from good old reliable Dodd, Mead. This is the 65th volume since the present publisher took over the series, and the 20th edited by Mr. Guernsey. With small extensions, it follows the recent trend. Besides the usual Broadway summaries and statistics, there are sections on Broadway, Off Broadway, Off Off Broadway, and the resident professional theater. The yearbook is illustrated with photographs plus some 50 pages of lively Hirschfeld drawings.
The ''Best Plays'' synopsized from last season's meager crop are ''And a Nightingale Sang . . .,'' ''Fool for Love,'' ''Painting Churches,'' ''Beckett Plays,'' ''Noises Off,'' ''The Miss Firecracker Contest,'' ''Glengarry Glen Ross ,'' ''The Real Thing,'' ''La Cage aux Folles,'' and ''Sunday in the Park with George.'' Signs of the times: Two of the ''best plays'' were musicals, four were imports, and seven represented the work of not-for-profit theaters.
We may disagree at times with Mr. Guernsey's choices. But whether for browsing, research, or argument-settling, ''Best Plays'' is one volume neither theater buffs nor professionals could do without.