Question: Are novelists Walter Van Tilburg Clark and Billie Letts both American? 

Context paragraphs:
Jo Anne Van Tilburg: Jo Anne Van Tilburg is an American archaeologist best known for her research on the statues of Easter Island (Rapa Nui).  Her primary specialty is rock art.

Where the Heart Is (2000 film): Where the Heart Is is a 2000 American romantic drama film directed by Matt Williams in his film directing debut.  The filmstars Natalie Portman, Stockard Channing, Ashley Judd, and Joan Cusack with supporting roles done by James Frain, Dylan Bruno, Keith David, and Sally Field.  The screenplay, written by Lowell Ganz and Babaloo Mandel, is based on the best-selling novel by Billie Letts.  The film follows five years in the life of Novalee Nation, a pregnant 17-year-old, who is abandoned by her boyfriend at a Wal-mart in a small Oklahoma town.  She secretly moves into the store, where she eventually gives birth to her baby, which attracts media attention.  With the help of friends, she makes a new life for herself in the town.

Walter Van Tilburg Clark: Walter Van Tilburg Clark (August 3, 1909 – November 10, 1971) was an American novelist, short story writer, and educator.  He ranks as one of Nevada's most distinguished literary figures of the 20th century, and was the first inductee into the 'Nevada Writers Hall of Fame' in 1988.  Two of his novels, "The Ox-Bow Incident" and "The Track of the Cat," were made into films.  As a writer, Clark taught himself to use the familiar materials of the western saga to explore the human psyche and to raise deep philosophical issues.

The Ox-Bow Incident (novel): The Ox-Bow Incident is a 1940 western novel by Walter Van Tilburg Clark in which two drifters are drawn into a lynch mob to find and hang three men presumed to be rustlers and the killers of a local man.

Track of the Cat: Track of the Cat is a 1954 film directed by William A. Wellman and starring Robert Mitchum and Teresa Wright.  The film is based on a 1949 adventure novel of the same name by Walter Van Tilburg Clark.  This was Wellman's second adaptation of a Clark novel, the first being "The Ox-Bow Incident".  "Track of the Cat" was produced by John Wayne and Robert Fellows for their Wayne/Fellows production company.

The City of Trembling Leaves: The City of Trembling Leaves (1945) is a novel in the semi-autobiographical genre, by Walter Van Tilburg Clark.  The novel is a series of parallel narratives detailing the lives and work of a group of redacted characters named Tim Hazard, Lawrence Black, Mary Turner, Rachel Wells, Marjory Hale, "Walt", (the narrator) and assorted other side-characters.

Van Tilburg: Van Tilburg is a surname.  Notable people with the surname include:

Where the Heart Is (novel): Where the Heart Is is a 1995 novel by Billie Letts.  It was chosen as an Oprah's Book Club selection in December 1998.  A 2000 film of the same name was directed by Matt Williams, starring Natalie Portman, Ashley Judd and Stockard Channing.

The Watchful Gods and Other Stories: The Watchful Gods and Other Stories is a collection of short stories by Walter Van Tilburg Clark published in 1950.  It brings together eight stories and one novella (the title story).  Three of the stories had already appeared in the annual anthology of O. Henry Award-winning stories, most notably "The Wind and the Snow of Winter" which was selected by that anthology, in 1945, as their "first-place winner."  Since this book's publication, two other stories have remained notable: "The Portable Phonograph" and "Hook" have both been widely anthologized since they were published.

Billie Letts: Billie Dean Letts (née Gipson; May 30, 1938 – August 2, 2014) was an American novelist and educator.  She was a professor at Southeastern Oklahoma State University.

Supporting paragraphs: Walter Van Tilburg Clark Billie Letts
Answer: yes