You can't judge a town library by its cover - the treasures may be hung as well as shelved

A souped-up car full of teen-agers screams by lime-green triple deckers and turn-of-the-century brick buildings, and then past the best-kept secret in this white- and blue-collar city of 55,000 people just north of Boston.

Behind the massive, brown sandstone walls of Malden's public library and up a short flight of stairs are two elegant, wood-paneled galleries casually hung with paintings by Winslow Homer, Joseph M. W. Turner, and Jean-Francois Millet, among others.

In Malden?

Like many New England industrial towns in the 19th century, Malden did well by its leading family, the Converses of the rubber shoe company. The Converse Memorial Building (designed by Henry Hobson Richardson, a noted New England architect) was built in 1885 to house the library and a municipal art gallery, in memory of the Converses' son, who was killed in an 1863 bank holdup. The family also set up endowments for the building's maintenance and the purchase of art.

Through the years, the library quietly acquired the 100 paintings that now fill the two galleries.

The collection ''is completely unexpected, the quality superb,'' says Morton Bradley, a well-respected art restorer from Arlington, Mass. ''It's a pity no one knows about it.

''It was picked by the best - Arthur Pope,'' Mr. Bradley says. (Mr. Pope, the noted color theorist, was professor of art at Harvard University from 1906 to 1949 and consultant to the library for 30 years.)

''He had a great eye,'' says Bradley, who was Pope's student for 40 years, ''and the collection, for the most part, is a mirror of his eye. He was uninterested in 'who' - just quality. And he was proud of those pictures at Malden. Anything that was beautiful was his field.''

Bradley says all the paintings Pope chose had to be passed by the library's board of trustees. ''One vote could kill a painting, and there happened to be one man who didn't like Impressionists.'' As a result, ''they turned down some lovely paintings,'' Bradley says with a sigh.

Nonetheless, Pope chose many fine paintings, including several by late 19 th-century American artists such as Frank Benson, Thomas Hill, George Inness, and Charles Davis, very much in vogue in art circles today - ''all the stuff that good Yankees with money will maybe spend some money on,'' according to one Boston art dealer.

This has not been lost on what Bradley and others in his field call the ''slick operators'' from New York art galleries, who canvass libraries, schools, and churches, ''trying to talk them out of paintings.'' And some institutions, Bradley says, ''are desperate for money.''

''Right now,'' he says, it is really a seller's market. ''There is a great shortage of good paintings.''

Why then haven't galleries gobbled up paintings from places such as Malden?

''I don't know how these places protect themselves,'' Bradley says. ''Perhaps it is a matter of local pride. But it would be a great mistake to sell anything. If one is sold,'' he says, ''you devalue the rest. It is worth more as a collection.''

Harry Reinherz, a Malden lawyer and president of the library's board of trustees, says: ''Last month as a matter of fact, some people from New York came to see the (Winslow) Homer (The Whittling Boy), and were surprised we had the (Frank) Benson (Hilltop). 'How come you people have it? How come you have it?' they kept saying. They thought the Benson was worth more than the Homer - but they made no offer.''

Mr. Reinherz says the trustees have never thought of selling anything. ''We see ourselves as custodians for the public of Malden, in a miniature museum,'' he says. ''But if someone offered a price beyond our wildest dreams of what we think something is worth - such a fabulous price might be considered. But as of now, that hasn't happened.''

Malden's librarian, Dina Malgeri, says that ''the last time there was any interest in the library was when the Homer was stolen (and recovered) in 1975.'' She plays down the latest inquiries. ''Well, some people are interested - in the Turner (Seascape), and the Homer. But those have always attracted attention.''

Morton Bradley says the real beauty of the library's collection is its size:

''If you go to the Metropolitan Museum of Art in New York, you see lots of things and don't remember a thing. It's just a blur. . . . You get picture fatigue.

''It is very important to see different pictures in small quantities. That's why Malden is such a gem.''