LONDON — Michael Crichton was the mega-selling thriller writer behind “Jurassic Park,” “The Andromeda Strain” and TV series “ER.” He also was a private and passionate art collector who bought works by 20th-century masters including Pablo Picasso, Andy Warhol and Jasper Johns.

Four pieces from Crichton’s collection valued at 20 million pounds ($32 million) — including an iconic Johns Stars and Stripes “Flag” painting that once hung in the writer’s Beverly Hills bedroom — went on display today before a scheduled May auction by Christie’s in New York.

The centerpiece is “Flag” of 1960-1966, one of a series of images of the U.S. standard by Johns that transformed the art world — challenging the supremacy of abstract expressionism and paving the way for pop art’s obsession with boldly colored found objects.

“It’s truly the first great pop art object,” said Brett Gorvy, deputy chairman of Christie’s, Americas. “It allowed artists like Andy Warhol and Roy Lichtenstein to look to the world around them.”

Crichton — who died of cancer at the age of 66 in November 2008 — was one of the world’s most commercially successful writers, whose books and films blended science and paranoia in a popular, populist brew.

In private, he was a knowledgeable art aficionado who often befriended the artists he collected. He was such an authority on Johns that the artist asked him to write the catalog for a 1977 retrospective.

“I never really cared whether a particular piece was major or minor, typical or atypical of the artist’s work, or whether the artist was fully or thinly represented in my collection,” Crichton once wrote. “I just bought images that I enjoyed looking at.”

Gorvy said that after Crichton’s death even some of his friends were surprised to learn of the size and scope of his collection.

“He never mixed the Hollywood world and the art world,” Gorvy said. For Crichton art “was an escape. He had a very physical connection to the art.”

Crichton bought “Flag” directly from the artist in 1974, and it has not been offered for sale since.

Christie’s sale also includes Picasso’s 1961 painting “Femme et Fillettes” (“Woman and Children”) and works by Robert Rauschenberg and Roy Lichtenstein from Crichton’s collection.

Christie’s is waiting for the outcome of its London contemporary sales next week to set individual price estimates for the works.

The last Johns flag painting sold at auction fetched $12.1 million in 1989 — before the art market’s boom, bust and current tentative recovery.

Rare works are once again selling for record prices. On Wednesday, Alberto Giacometti’s sculpture “Walking Man I” sold at Sotheby’s for just over 65 million pounds ($104.3 million), the highest price ever paid for an artwork at auction.

“As we’ve seen this week, when you’re dealing with things that are the most precious objects and extremely rare, you get a whole cross-section of buyers,” Gorvy said.

The works are on display until Feb. 12 at Christie’s in London and will be sold May 11 in New York.