WHAT CAN YOU say about a book on the rights of people in the armed forces? "It must be a very short book," said one wit quoted in the introduction to The Rights of Military Personnel. By Robert S. Rivkin and Barton F. Stichman (Avon-Discus, $1.50). Actually, it turns out, military personnel have quite a few rights, including one that the authors get to immediately (in capital letters): "THIS BOOK MAY NOT LEGALLY BE TAKEN AWAY FROM YOU. It may be exmained by your military superiors, but it should be returned immediately. (See Chapter Six, First Amendment Rights.)" They add a recommendation of "caution in the assertion of rights . . . as there is no assurance that at any base at any particular time the authorities will handle a situation in the most rational way."

The book is one of more than a dozen, prepared by the American Civil Liberties Union and publish by Avon, presenting in readable, no-nonsense style the rights of a variety of American sub-groups. Least expensive is The Rights Of Prisoners (95 cents); women and gay people have to pay $1.75, while reporters, mental patients, teachers, students, aliens and young people, among others, occupy a middle ground between $1.25 and $1.50. The prices may reflect the quantity and complexity of the rights of the groups in question - or, more concretely, the amount of paper and ink needed to discuss them.

The volume on military personnel is clear and useful on such intricate subjects as selective conscientious objection and the vital question: "Can a military order to do something impossible be legally enforced?" Let's Define Our Goals

There are 702 pages in The Encyslopedia of Football: Fifteenth Revised Edition, by Roger Treat, revisions edited by Pete Palmer (Doubleday/Dolphin, $6.95), which weighs it in at just over a penny per page - expensive compared to the paperback edition of Trinity, which offers nearly three pages for each penny but will not tell you who started at quarterback for the Boston Yanks in 1946. Besides history, statistics, endless lists of players and a lot of nostalgic pictures, this volume has two pages explaining referees' signals and seating diagrams of all the major stadia. Back in Paper

Listed below, in no particular order, are some former hardcover books whose paperback appearance you may have been awaiting.

The Picture History of Photography: From the Earliest Beginnings to the Present Day, by Peter Pollack (Abrams, $8.95). Photography must be viewed as a technical process whose history is largely one of constantly improving equipment and the growing range of opportunities open to practitioners. Or it may be viewed as an art, reflecting the unique visions of a series of artists who have used lens and film as Rembrandt used canvas and pigments. Pollack accommodates both views in this succinct but thorough survey, but his emphasis is heavily on the art of photography as reflected in the work of more than 30 manor photographers in the last 150 years. The text is thoughful and well-written, but the book's strongest attraction is its use of 257 illustrations - 18 pages in color - reproduced with the technical excellence one expects from this publisher (see illustration).

The Black Family in Slavery and Freedom: 1750-1925, by Herbert G. Gutman (Vintage, $6.95).

The Fancy Dancer, by Patricia Nell Warren (Bantam, $1.95.). Male homosexuality as a fictional theme has reached mass-market status, and Patricia Nell Warren (also the author of the widely acclaimed The Front Runner is its incongruous but sensitive prophetess.

J.F.K.: The Man and the Myth, by Victor Lasky (Dell, $2.75). It didn't start with It Didn't Start with Watergate.

Economists at Bay: Why the Experts Will Never Solve Your Problems, by Robert Lekachman (McGraw-Hill, $3.95). Doctors' mistakes are buried and lawyers' go to jail; had happens when economists are wrong (at least the influential ones) is inflation or high unemployment or, most recently, bot at once. Lekachman, an economist himself, diagnoses current economic problems as largely iatrogenic.

First, You Cry, by Betty Rollin (Signet, $1.95). The NBC News correspondent's story of her encounter with cancer.

Past Forgetting: My Affair with Dwight D. Eisenhower, by Kay Summersby Morgan (Bantam, $2.25). With photos.

FBI, by Sanford J. Ungar (Atlantic/Little, Brown, $7.95).

The Superwives, by Jeanne Parr (Avon, $1.75).

Casey at the Bat, by Ernest Lawrence Thayer; illustrated by Jim Hull, with an introduction by Martin Gardner (Dover, $2.50). If 4.8 cents per line seems a bit expensive for a 52-line poem, even one as beloved as this, consider the line-drawings (56 of them; some half-lines obviously demand a picture all their own) and the informative introduction, and you may conclude that Dover has cleared the bases.