Thurber's theory; 'Laughter improves everything'; Selected Letters of James Thurber, edited by Helen Thurber and Edward Weeks. Boston: Atlantic-Little, Brown & Co. 269 pp. $15.

James Thurber deftly took care of the whole business of collecting authors' letters when he reviewed an imagined volume of his own correspondence. ''A certain Groping, to be sure, is discernible,'' he wrote, ''but it doesn't appear to be toward anything.'' Used as a Foreword to this book of actual Thurber letters, his critique demonstrates in spades the vision of humor that glints through the following pages - a vision undiminished by the impaired eyesight that looms large in the letters but not without an edge of humorous perspective, too.

''Laughter need not be cut out of anything, since it improves everything,'' Thurber writes. ''The power that created the poodle, the platypus, and people has an integrated sense of both comedy and tragedy.'' He goes on to give an example of taking humor seriously, as all good humorists must, even while fighting the impulse to take themselves too much so.

''(Robert) Benchley once said, 'Only a humorist could take humor apart, and he has too much humor to do it.' Serious definition of a free-lance writer: One who gets paid per word or per piece. Benchley's humorous, hence perfect, definition: 'One who gets paid per word, per piece, or perhaps.' Which is the more serious, the utterly serious, or the partly humorous?''

Hence, no doubt, Thurber's regard for research, for getting things right as well as funny. When, for instance, he writes his celebrated book about editor Harold Ross and the New Yorker, he fusses in letters about not being able to set down everything that he would like to. The letters add to the Thurber legend of hard writing making easy reading, the man whose first drafts ''sound twelve years old,'' whose supple prose is the product of multiple rewrites.

Yet Thurber is, or lets himself appear, as human as the characters in his humor. He admits to being depressed when, amid all the lines attributed to New Yorker writers, ''something of my own, of which I was fond, was never repeated to me by anyone.'' It was when someone asked him to draw new illustrations for ''Alice in Wonderland'' and he replied, ''I tell you what let's do, let's keep the Tenniel drawings and I'll rewrite the book.'' He also rather liked the sign he put up at the office when partitions were being constantly changed: ''Alterations going on as usual during business.''

The word ''selected'' really means something in the title of this volume. We're left wanting more missives to editor Ross, for instance, who gets fewer pages than Thurber's eye doctor. We wish we could see the omitted letters, of which there were tantalizing glimpses in Burton Bernstein's biography of Thurber. There is a lot of white space in addition to the still incomparable Thurber drawings.

But the selected morsels are often hilarious or touching. Here is the comic technician debating getting extra mileage from cartoons by finding new captions for them. Here is the friend concerned with matters far from the printed page, whether or not the correspondents are writers, too, like Katharine and Andy (E. B.) White.

Thurber admires Sinclair Lewis for being nice to his secretary's elderly mother. He delights in being able to write ''that hardest of all things, a letter to a girl six years old'' (if only one of those were included!). He defends John O'Hara's small-boyishness, for unless artists ''can remember what it was to be a little boy, they are only half complete as artist and as man.''

''Every time is a time for humor, especially now,'' he writes to Andy White in the early 1950s, ''because the Communists set out long ago to knock it off, and writing it is doing battle in one small corner of the field.''

Thurber's corner has turned out to be not all that small, and it is good to have these letters, so congenially edited by Helen Thurber and Edward Weeks, to fill it a little fuller.