"Hey--please return a book! I want to read--Thanks," said the graffitti in the Porter Square T-stop.

The graffitti was scrawled over a bookshelf in the station--a shelf that is also the only unsupervised public library in the country. There is no librarian, no checkout, no catalog. Passengers may take books off the shelf at any time, read them, and return them at their convenience.

The program, known as "Read, Ride, Return," is a joint effort of the Massachusetts Bay Transit Authority (MBTA) and the Massachusetts Board of Library Commissioners.

The shelf is part of a Cambridge Public Library project to extend its services to even the busiest of business folks. Located on the inbound platform near the escalators, the case is a little library for people who want something to do while they commute.

The existing program also maintains a shelf at the Division of Employment Security in Cambridge, where unemployed people often have to stand in line for hours to pick up their benefit checks.

It is the first project of its kind in the state, according to its inventor, Ron Brown, the associate director of publicity at the Cambridge Public Library. Although other commuter libraries exist in Toronto and Decatur, Georgia, this is the only one that relies purely on trust.

Since the program started six months ago, Brown said he has spent part of each day replenishing the supply of books, to make sure that the collection has something for everyone. But he said he had not put out books for the last month.

"It looks like it will be self-sustaining," he said, adding that he hoped further funding would not be necessary to perpetuate the program.

Last year the Cambridge library received a $15,000 grant from the state Board of Commissioners to create the service. Some of that money went to build the shelves to the MBTA's strict specifications. Spokesman Vincent Carbona said MBTA officials wanted to ensure that the bookcase would fit in with the station's general appearance and would not cause a safety hazard.

But neither aesthetics nor safety caused grave problems. The library persuaded Paul Dietrich, the architect who designed the station itself, to design the bookcase at no cost. The library then paid for the materials and labor needed to build the shelves. "The designer knew what he was doing," said Carbona. "He put the shelf in the right location, and it looks beautiful."

The minature library has already developed a dedicated clientele. When it was first installed, "people were just standing here staring at it," said Mark E. Carlson, a computer consultant who said he now uses it regularly. Calling the library "socialism come to the T," Carlson said riders now use the service so often that it is sometimes hard to find books.

Although some riders said they assumed theft was a problem, most said they had donated more paperbacks than they kept.

"I don't keep the books, except a couple that I really like," said systems programer Liz C. Marshall, hastily adding that she "brought a lot more" to appease a guilty conscience.

Another rider acknowledged that she usually returned books "at a slower rate than I pick them up." She said she did mean to return all the books--eventually.

Another passenger said he did not borrow from the shelf because "I like to own books." He said he would want to keep the books, if he took them to read during his commute.

"Mostly I leave New Yorkers because I hate to throw them away," said another Red Line user, adding that she did not use the library because "other people might not have anything to read, while I always have New Yorkers."

Other passengers said they like to donate books that they no longer need or that they want others to read. They said they would rather leave them on the Porter Square shelf instead of throwing them out or giving them to friends. Brown said one young man who brought 13 boxes of paperbacks to his office for the program.

The program does not have universal appeal. "I don't read. I watch T.V.," said one elderly man. "At least I didn't tell you I can't read," he added.

Despite Brown's efforts to provide a varied selection of books, many passengers complained that all they ever find on the shelves are romances and mystery novels. One commuter described the service as "a trashcan for people's old books."

Brown said he did not know how many of the books actually came back, but guessed that readers returned "at least 50 percent." But because of the many donations, he said theft was not a problem.

And plans are already under way to expand the program. Robert Dugan, a commissioner involved in the project, said that of the 270 grants his organization examined, "this is the one we keep talking about." He said he would be happy to consider expanding the program in Cambridge or even as far as Boston.

Brown said he hoped to put another shelf in Somerville's Davis Square station, and eventually to put a staffed kiosk in the Porter Square station. But he said Harvard Square was not likely to get a shelf because it would be hard to stock such a busy station.