President Carter emerges as an astute reader and incisive literary critic in the course of a brief appearance in "Agee," an abosoring biographical documentary about writer James Agee shown at a special preview last night at the American Film Institute Theater.

Learning of Carter's admiration for "Let Us Now Praise Famous Men," and exalted work of personal, metaphysical journalism that grew out of Ageehs assignment to do an article about Southern tenant farmers for Fortune magazine in 1936, filmmaker Ross Spears arranged an interview in Plains in the summer of 1976. Asked about Agee's journalism, Carter sums up eloquently: "I believe he added a new dimension to descriptive literature."

The appreciation is equally applicalbe to Agee's film criticism, which added richly satisfying dimensions of analysis, style, humor and emotional commitment ot a generally sterotryped, derlict branch of literature.

(I must have been one among thousands of avid, blindly aspiring young movie nuts bowled over by the publication of "Agee on Film" in 1958.)

Spears has assembled a filmed chroncile of Agee's literary career, which began at Harvard in the early '30s and ended prematurely in 1955, when Agee suffered this third heart attack and died in a New York taxi at the age of 45. Considering his previous lack of experience and limited budget (about $50,000 including an initial grant from the Tennessee Arts Commission), Spears has produced an appealing and informative memory album.

At the same time, the film isn't inquisitve or imaginative enought to recommend itself to viewers unfamiliar with the subject. Spears permits himself some ill-advised liberties, dramatizing a few episodes in pantomime that would be better illustrated and reinforced by authentic and evocative photographs. At one point Spears himself even "plays" Agee, wlking throughfully around a tenant shack while passages from "Let Us Now Praise Famous Men" are read on the soundtrack.

This form of hoage ought to be studiously avoided. Quotations cry out for a sustained, rhythmic integration of the illustrative material that originally enhanced Agee's text -- the Walker Evans photographs for "Let Us Now Praise Famous Men," a few of which are astutely inserted at other points.

Agee's widow and his two ex-wives look careworn and haunted in eerily similar ways, suggesting that his life and death took a toll on them that is not openly acounted for in their discreet recollections. It's a shame that there are no early photos of Agee with his first wife, the former Olivia Sanders, since the contrasts provided by intercutting old snapshots with interviews of the subsequent wives, Alma Mailman Neumann and Mia Fritsch Agee, are quite startling.

It also would have been nice to see pictures of Agee's children (a son by Alma and two daughters and a son by Mia) and of Robert Fitzgerald, Dwight MacDonald, and John Huston, three of Spears' principal interview subjects at the time they were agee's intimates or collaborators. The fitfull dramatizations were probably easier for Spears to manage, but the material demands documentary images to compensate for the quotation-heavy sound track.

Nevertheless, Spears has done a decent, gratifying job of celebrating his own immoderate affection for James Agee. There are revealing bits of testmony from wives and friends, or moments when the inherent eloquence of the quotations justifes Spears need to read aloud from a favorite author.