It’s easy to describe Jean Strouse’s latest publication. It’s a beautifully written book about the connections between the painter John Singer Sargent and a family he painted often, the Wertheimers. What’s less easily summarized is what most interests Strouse about her chosen subject.

“Family Romance: John Singer Sargent and the Wertheimers” by Jean Strouse. 311 pages. $32 Farrar, Straus & Giroux

One good answer would be: everything, which is why this is a book and not an article. Another answer, less magnanimous, might be: slightly too much. Too much in the sense that “Family Romance: John Singer Sargent and the Wertheimers” is stuffed with so much information that, like the Wertheimer family tree, it spreads with a sort of blind, vegetal energy and frequently loses focus. Apropos of not very much, we learn, for instance, that Ronnie Wood (of the Rolling Stones) collects the paintings of William Orpen; that when the artist Ronald Searle moved into a modernist house in Paddington he found paintings by Lucian Freud left behind in the cellar; that Highclere Castle in Hampshire was the setting not only for “Downton Abbey” but also the orgy scene in Stanley Kubrick’s “Eyes Wide Shut”; and that the British politician Philip Sassoon was once chased around a college quadrangle with a bullwhip.

But I think, having registered it, we can bury this ungenerous objection. Focus is overrated. And if you have something interesting to say that’s not quite on topic, why not say it? Strouse is the author of an acclaimed biography of Alice James, the American diarist whose brothers included Henry and William James, and another of the financier J.P. Morgan – known as “Napoleon of Wall Street.”

Readers (and they are legion) interested in Sargent (1856-1925) – his peripatetic life, his array of rich and accomplished friends, his fluctuating critical reception – will want to dive into “Family Romance.” Likewise, anyone interested in the Wertheimers, a British family of German-Jewish descent who played a major role in art and culture in the late 19th and early 20th centuries, will find it absorbing.

Over several decades, the Wertheimers were Sargent’s most important private patrons. Asher Wertheimer, an art dealer, first commissioned Sargent to make portraits of himself and his wife to mark their 25th wedding anniversary in 1898. Sargent went on to make many more portraits of the family, forging close friendships with several of its members. “Apparently,” Strouse writes, “the artist had open invitations to Asher’s London townhouse and his country retreat in Berkshire.”

Asher left nine of these portraits to the National Gallery in London. His gift to the British nation was gratefully received and rightly celebrated, but also criticized, sometimes on grounds that seemed to betray an underlying antisemitism. The paintings were soon moved to the Tate, which acquired a 10th in 1996.

The portraits mostly languish in storage, but they were the subject of a 2001 exhibition, which Strouse happened to see at the Seattle Art Museum. (The show was organized by New York’s Jewish Museum, where it had opened in 1999. It also traveled to the New Orleans Museum of Art and the Virginia Museum of Fine Arts in Richmond.)

In Strouse’s account, artist and family are often uncoupled; we follow Sargent first, and then various Wertheimers. Along the way, the reader is treated to disquisitions on the decline of the British aristocracy and rise of the American robber barons, the role of art collecting, the machinations of art dealers, antisemitism, orientalism and the minutiae of estates, family trees and finance (Strouse knows, as her book on Morgan showed, how to follow the money).

One of the book’s consistent themes is the fluctuation of reputation. Styles of painting and success in business can both fall victim to prejudice, misfortune or simply diminished interest.

The Wertheimers, especially Asher, were fabulously successful. Asher earned the confidence of the Rothschilds, the British royal family, and clients and dealers in Britain, Europe, Russia and America. Beginning with Asher’s father, Samson, who had started the family business, the Wertheimer name was central to the formation of many of the most important art collections in the late 19th century. But by 1920, the Wertheimers were well on the way to being forgotten.

Sargent, similarly, was the most sought-after portraitist on either side of the Atlantic. But after a hugely popular memorial exhibition in 1925-26, he came to be seen as irrelevant, passé and ideologically suspect (having painted primarily the wealthy). He wasn’t the subject of another museum exhibition in Europe or America for three decades. Only recently have his paintings, watercolors and drawings come back into vogue.

In her introduction, Strouse writes that she had always associated Sargent “with portraits of British aristocrats and Boston Brahmins, not with Jews.” Her subsequent investigations show that he painted 35 portraits of Jewish sitters, plus two watercolor portraits and 37 portraits on paper (most of them in charcoal).

As she embarked on her book, Strouse wondered whether, for all their astonishing professional success, Sargent and Asher Wertheimer had been “drawn together in part by shared outsider status.” Sargent, she points out, was an American expatriate, lifelong bachelor (likely gay) and a “cosmopolitan nomad.” Wertheimer, meanwhile, “lived in a culture that regarded Jews as ineradicably ‘other,’ and art dealing, no matter how illustrious, as ‘mere trade.’”

While “Family Romance” suggests there is truth to these speculations, Strouse complicates the picture with deft accounts of both Sargent’s and the Wertheimer family’s tribulations beyond these questions of identity. Asher Wertheimer was burned by his dealings with another storied art-dealing firm, Colnaghi, whose “machinations,” Strouse writes, “effectively sold” a very valuable collection of gems “out from under Asher’s clients,” with the result that Wertheimer’s role in this significant deal has been obscured.

Characterizing Wertheimer as “more scrupulous” and “less aggressive and self-promoting” than his competition, Strouse argues that he was “unprepared for the brutal tactics of the modern competitive marketplace.” He was also harmed when he did too much work on credit for a French aristocrat named Count Boniface de Castellane. When Castellane and his American wife, a railroad heiress, refused to pay for what they had acquired, Wertheimer was forced to sue. A scandal erupted. Castellane countersued. Wertheimer eventually won, but the protracted lawsuit and attendant publicity were a “nightmare,” according to Strouse. He changed his approach, henceforth “financing deals rather than actively participating in them.”

Sargent’s suite of portraits could persuade anyone that the Wertheimers were the happiest, brightest and liveliest of families. But of course, no family is ever like that. Strouse’s accounts of Asher’s conflicts with his brother Charles; of his son Alfred, who died of a morphine overdose at the age of 26; of another son, Edward, who died on his honeymoon in Paris after eating a bad oyster; and of his daughter Ruby, who died in fascist Italy in 1941 after being denied the health care she needed because she was Jewish, are all reminders of life’s frighteningly fragile foundations.

Another daughter, Ena, of whom Sargent made four portraits, is the subject of a vibrant chapter that includes accounts of the Russian impresario Serge Diaghilev and his Ballets Russes circle.

Sargent’s own trials are not ignored. The scathing response to “Madame X,” his portrait of the young socialite Amélie Gautreau, which was seen by the French as indecent and socially grasping, is described early on. Some of the later criticisms of Sargent by such modernist critics as Roger Fry were fired, according to Strouse, by strains of antisemitism.

But of course, Sargent was not himself Jewish, and we learn toward the end of Strouse’s account that he got himself, in his own words, “in hot water here [in Boston] with the Jews.” He had accepted a mural commission from the Boston Public Library, which he worked on for nearly three decades. The narrative cycle, called “Triumph of Religion,” ended with allegorical figures representing Church and Synagogue. Where the Church was represented by a beautiful young woman, the Synagogue was, as Strouse writes, “a blindfolded crone, slumped to one side amid ruins, her crown falling off.” Sargent’s use of this “derogatory Catholic icon for ‘Synagogue’ is especially remarkable,” Strouse writes, “since, not to coin a phrase, some of his closest friends were Jewish.”

Nonetheless, Strouse is clearly sympathetic to Sargent as a man, and she mounts a rousing defense of his art. Against the idea that he was essentially a gun-for-hire and servant to the rich, she argues that his work is more multivalent in its modernity. It expresses, she writes, quoting the art historian Elizabeth Prettejohn, the “dramatization of social flux as opposed to the petrification of social status.”

You feel in good hands reading Strouse. She is measured, her research is impeccable, and she tells of interesting lives in interesting times with interesting footnotes to boot.