It is a fine time for connoisseurship in New York. One of Raphael’s last pictures has been on loan to the Frick Collection. The Fra Carnevale show at the Metropolitan Museum is the sort of event that usually requires transatlantic travel. And there’s a delightful grouping of 15 old Italian paintings at Wildenstein on 64th.

There are no masterpieces in the gallery show – nothing obviously by one of the masters of the quattrocento or the cinquecento – but there is much to delight in, particularly as none of the pictures are under glass.

A torso of Christ (“Man of Sorrows” [c. 1485]) by Raphael’s father, Giovanni Santi, is an immediate reminder of the Netherlandish influence that made itself felt in Italian art in the last half of the 15th century. Here is Memling and Dieric Bouts in full force, with that thick, smooth buildup of paint for skin, on which shockingly red drops of blood are dripped. The figure’s placid face, eyes closed and softest of smiles, emphasizes the true violence of the stigmata, the spear wound, and the crown of thorns.

What is marvelous are the changes rung on the triangle shape, beginning with the nose – Christ’s face hangs slightly to the left. The angle is echoed by the diamonds under his eyes and those created by his split beard, his clavicles, the position of his hands and fingers, and by the folding of his arms on the chest. It is a bravura composition, one that recalls Bouts’s “Man of Sorrows” (c. 1460– 75).The Bouts original is lost, but there are numerous copies (I know of at least one in the Met, coupled with a mourning virgin, and two in the National Gallery, London.)

A more accomplished painting is a Madonna and child (c. 1510) by another of Raphael’s early masters, Luca Signorelli. The mother, in profile, is exquisite. Her delicate face, carefully flowing gown, and filmy mantle are Luca at his finest. Her left hand, though, is awkward and stands wrongly in the picture’s center. The whole composition is too flat and static, with the mother’s solicitude for child never coming clear. Perugino and Raphael would solve such problems dramatically.

The child shows the Netherlandish influence in his folded flesh, very much a northern trick in that era. He is cousin to the child in the magnificent Williamstown Piero della Francesca in the Met’s “Carnevale” show. You can see what Luca learned from Piero – he trained in his workshop – and how his paintings looked forward to the rapid achievements Italian art would make in the next generation in Rome.

All of the show’s works offer some delight. There’s a large Piero di Cosimo illustrating a scene from the “Argonautica,” though its condition is rough, with the exception of the queen’s superb face. An Ercole de’ Roberti (or at least a School of) boasts a riveting Mary with an angelic face and fold upon fold of overflowing blue robe.

To cite just one more work, the Lorenzo Veneziano “Birth of St. John the Baptist” (c. 1350) is magnificently odd. It is structure upon structure: From ornate frame to gold-leaf background to the building to the bed platform, a puzzle of architecture is created. What stands out most though is the tiny bed of the Baptist. It lies empty as the child nestles in his mother’s arms in the parallel large bed. Still, the delicacy of the fringe on the blanket and the white on white lace of the pillows is marvelous.

Such are the pleasures of connoisseurship, but also the questions. I don’t know what to make of the queen’s face in the Piero di Cosimo. It is so much sweeter in preservation than the rest. Was it repainted, or just luckily preserved? Knowing the answers to such questions is what makes an connoisseur, and they are the subject of an excellent new book edited by the New York lawyer Ronald D. Spencer: “The Expert Versus Object: Judging Fakes and False Attributions in the Visual Arts” (Oxford University Press, 241 pages, $35).

Mr. Spencer has pulled together a sequence of essays that explain how fine art is attributed and evaluated – expert opinion, provenance, catalogue raisonnes, scientific examinations, the role of intuition, signature identification, and the legal liability for experts. The basic questions are answered, the lines of inquiry are laid out, and the notes will send the curious to scholarly tomes. Many of the contributors are well-known artworld figures: E.V. Thaw, Samuel Sachs, Peter Sutton, Theodore Stebbins, and the owner of Ursus Books, Peter Kraus. I was particularly impressed by the clear, rapid presentation of scientific evaluation methods by Rustin Levenson.

The great German scholar of Netherlandish art Max Friedlander once said that to attribute a fake as real is a mistake, but to call a real work a fake is a sin. This generosity is one of the keys to keeping connoisseurship out of the hands of the self-elected few. Mr. Carter’s contributors are closer in spirit to Friedlander – an excerpt of whose 1919 “On Art and Connoisseurship” is included – than to the caricature of the condescending art snob found in prints by Hogarth and Rembrandt and in innumerable novels. Connoisseurship is simply paying careful attention to what you are looking at. Build a large visual memory, read the scholarly works, and you will be on equal ground with any expert.

Still, it is interesting how rare it is for a great collector to be a great connoisseur. The only two American examples that come to mind are Duncan Phillips and John G. Johnson. Phillips’s museum in Washington is a glorious treasure-house. Johnson’s collection is harder to know, as after a few years of being partially maintained in his South Broad Street mansion, it was brought into the Philadelphia Museum of Art in 1933 and then folded into the main collection in 1995. “Folded in” may not be the right word, considering that of the 469 pre-1800 Italian paintings in the PMA, 457 of them are from the Johnson Collection. (He left 1,279 art objects – including a Duccio, a Masaccio, a Campin, Botticellis, and Giovanni di Paolos – to the city when he died in 1917.)

Johnson didn’t have the financial resources to compete with the Wideners, Morgans, and Kresses and so focused on early Italian art. He bought far fewer fakes and copies than his richer contemporaries, taking a serious scholarly interest in what he was buying. He sought the friendship as well as the advice of people like Roger Fry and Bernard Berenson.

The heart of Johnson’s collection is the subject of the new book “Italian Paintings 1250-1450: In the John G. Johnson Collection and The Philadelphia Museum of Art” by Carl Brandon Strehlke (Pennsylvania State University Press, 556 pages, $95). Mr. Strehlke is the adjunct curator of the Johnson Collection at the PMA and knows his subject through and through. The entries on each artist – from Allegretto di Nuzio to Marchigian School (c. 1425-30), with one fake from 1900 included – are the usually informative catalog pieces, but the real interest comes with the tracking of the attribution of each work.

A beautiful Fra Diamante painting of three saints came to Johnson as a Verrocchio in 1905. Over the course of the 20th century, it was variously attributed as School of Fra Filippo Lippi, Francesco Botticini, Pesellino, “Pesellino (in great part),” and “Florentine c. 1460.” It is a Fra Diamante today, but for how long? Such is the essential history, murky as it is, of connoisseurship, and of taste.

The whole book is a model art-historical production. Now if only the Johnson Collection were a subway ride away, and there was a second volume on Italian paintings post-1450.

Until April 1 (19 E. 64th Street, 212-879-0500). Prices: $450,000-$1.5 million.