MENTION AN Atlantic crossing on board the Queen Elizabeth II to most people, and visions of the bounding main, salt air, long walks on deck, dining and dancing fill their head too, but soon uneasiness creeps in. Lounging on a deck chair under a great woolen robe is fine, but after contemplating the flat vastness of the sea for so many days, wouldn't one feel that familiar craving for a really good book?

Those were my apprehensions on such a trip recently as I carried the de rigueur satchel of books and magazines from my hometown library, sure that I'd have to assuage that craving independently. But as I read the ship's bulletin on the first morning of the voyage, a curious announcement caught my eye. The QEII, the ship's daily news sheet said, is the only ship in the world with a library staffed by a professional librarian.

As that's my profession in a public library outside Washington, I found myself curious, even anxious, to examine the books and their keeper. I was not a little envious of someone actually working as a librarian on the luxurious and glamorous Queen--so far from the suburban library I knew back home. Yet at the same time I doubted that this would be a "real" library.

So I ate a huge English breakfast and quickly went down to the quarter deck to prowl around the collection, prepared beforehand to "tsk-tsk" its shortfalls and ready to lend a professional hand.

What a surprise! Conveniently located in the middle of the ship off an airy lounge and bar, the library was a large wood-panelled room, with lots of comfortable seating completely occupied then and every time I returned. And in the midst of the books, people, magazines and video cassettes sat librarian Barbara Sants, pleased to be there.

Sants cordially explained the library and her presence there in a conversation later that evening. She was a reserved, small British lady whose blue eyes shone with intelligence and humor, with the complexion of a teen-ager that belied her years. "Pretty long of tooth," as she put it, Sants was called out of retirement last September after the Falkland war prompted Cunard Lines to spruce up their library on the Queen. A friend of Sants got the contract to supply the liner with a balanced, current collection of books and convinced Cunard that materials alone were not enough.

Up to that point the library was a shabby collection, overseen as on other passenger liners by a seaman. Most of the books were donated by passengers over the years who had been given "crossing" packets of bestsellers from New York bookstores. The personality of the crew member responsible for the collection would also be reflected in it, with lots of detective stories, "men's" adventures and other exotica. And many of the books were lost due to poor circulation control and the dilemma of the shipboard patron who reaches port while midstream in a good story.

All that has changed since September. Sants works with a collection that still contains gifts from debarking passengers, but that for the most part is supplied, processed and rotated by Special Libraries Book Service, an Oxfordshire firm.

What's in the collection? Mostly what you'd expect on a large luxury ship, with a few surprises. Most passengers are in their late 40s and 50s, with only a few children on board, so best sellers, fiction, travel and large print are very popular. The 3,000 books are arranged by category into foreign languages (including French, German, Japanese and Spanish), classics, fiction, children's (with many current picture books and older readers' paperbacks from both sides of the Atlantic), self-help, science, biography, religion, philosophy, nautical titles, travel (tailored to the ship's current destination), humor and reference. There is a card catalogue and an awkward classification system that Sants has not yet had time to make right. She's too busy circulating items and answering questions.

What does a ship's librarian get asked? Just about everything. Americans ask for all the best sellers, and the British passengers generally prefer weightier titles. Passengers are interested in the history and tourist attractions of their destinations. They also have the leisure to get into disagreements about peerage, trees, cricket, the formula for converting miles-per-hour into knots, and nautical trivia, and Sants must settle them all. She relies heavily on Whitaker's Almanac, the Encyclopedia Britannica, Who's Who's and the London telephone directory.

What kinds of books are most popular? Besides fiction and travel, money, dream interpretation, the history of the Titanic (for the sea-phobes), and--of all things--cookbooks. Apparently during a trip in which a major concern is finding the appetite to do justice to an overabundance of food, people still find it relaxing to browse through recipes.

Is loss of books still a problem? Yes, to some extent. A passenger who has paid thousands of dollars for the voyage may not feel much compunction to return "The Valley of Horses" just when he's reached the meeting of Ayla and Jondalar. Sants is trying to solve this problem by buying more short-story collections, which are frequently requested. Books are checked out by writing down title and last name and room number of the borrower in a log book. On the last night of the voyage she and a helper discretely slip overdue notices under cabin doors. The system works well, with 684 titles circulated during my five-day journey, and the library filled during open hours with all manner of passengers and crew.

Any special collections? Yes, games and a Time-Life series of video cassettes on religion, communications, dance, illness, explorers--48 separate tapes that can be viewed on the four televisions in the library, the only ones on board.

Is it fun? Definitely, except during heavy seas when no one has fun, and while the ship is in port. The turn around time is very short, usually less than a day, so passengers are shooed away as quickly as possible to let the crew spruce up the ship for the next lot. And Sants is busy checking stocks, logging in new books, and trying to do a little sightseeing on her own.

One of four children, Sants worked in a library during college, when World War II interrupted. Just as she was about to enter Britain's only library school, a German bomb destroyed the school, killing the teachers. She then joined the Royal Air Force and was assigned to repairing radios--a task she found herself unsuited for.

After the war, she passed her library certification examinations--something comparable to a masters degree--and worked briefly in a New Zealand library before returning to England to work at libraries in Oxford, and a country library "where I could look out my window to see lambs a'borning."

She finished her career with 19 years of service in British Petroleum's research library and retired to a small house in the country northeast of London. When the year-long Cunard offer came, "it was difficult to leave my home and beloved garden, but most of all my cat Alexander--whom I miss very much at sea." Still, the opportunity "to see the world and meet the most interesting people" proved too much. "When opportunity knocked, I answered."