ONCE UPON A TIME, it was all fantasy - and rather crude fantasy, as you can see by browsing through The Collected Works of Buck Rogers in the 25th Century, Robert C. Dille, editor; introduction by Ray Bradbury (A&W Visual Library, $8.95). Reality has in some ways bypassed the "Battle on the Moon" and "Tiger Men of Mars," which enliven these large pages of quaintly old-fashioned futurism. The world of Buck Rogers is upon us (ahead of scheduele) and it looks bewideringly both like and unlike what we saw long ago in the funny papers.

Nothing in all the Buck Rogers saga, for example, quite prepares the mind for the picture on page 117 of Space Colonies, edited by Stewart Brand (penguin, $5). The picture shows a spaceman relieving his blader in zero-gravity, and with it is an interview with an astronaut: "On Apollo the urine then would go outside, and you'd have to heat the nozzle because, of course, it instantly flashes into ice crystals . . . The most beautiful sight in orbit, or one of the most beautiful sights, is a urine dump at sunset . . . just a spray of sparklers almost."

In simple fact, the colonization of space (at least the orbital space not too far from our own planet) is already in he blueprint stage; mass-market books about it are beginning to appear, and actual work on building the first orbital colony could begin in a few years.

Stewart Brand's book is non-liner; it does not so much discuss as surround the subject, looking at it from dozens of angles and giving room to a variety of apparently conflicting views. Anyone who is turned on by the concept of space colonization will want to examine these ideas.

Colonies in Space, by T. A. Heppenheimer (Warner, $2.50), is readable and well-arranged, beginning at the point where it will find the average reader (skeptical and rather hazy on how-to) and proceeding logically through the possibilities and potential value of space colonization, examining the routine details of daily life in a space colony and ending up with some wild-eyed speculations about the remote future.

Both books lean heavily on the work of Gerard K. O'Neil, who began developing th ebasic concepts in 1969 as the teacher of a seminar at Princeton for specially talented science undergraduates. He tells it his own way in The High Frontier (Bantam, $2.75), with drawings by Donald Davis - a little drier and more detailed than Heppenheimer, but quite readable, and clearly the one book on the subject to get if you plan to get only one. For those who have not examined the idea before, any of these books should be a mind-expanding experience. Back in Paper

Leo Rosten's Treasury of Jewish. Quotations (Bantam, $2.95). This compendium (by the author of The Joys of Yiddish and the Hyman Kaplan books) includes 4352 quotations from sources that range from Genesis to Sholom Aleichem and are arranged under subject-headings ("Absolutism" to "Youth") with copious introductory and explanatory material, a glossary and an index. Many of the subject-divisions ("Cantors," "Divorce," "Gentiles," for example) are preceded by small, illuminating essays, which also include quotes from sources as diverse as Freud and Plato.

The Hi-Fi Handbook, by The Instute of high Fidelity (Ace, $1.95). if there is any "official" treatise on high fidelity, it is this one. It describes in readily understandable terms the kind of performace to expect from each component in a system and how it can be evaluated. It is impartial in the sense that it does not recommend any specific brand of equipment, but enormously (and properly) partisan on the issue of high vs. low fidelity.